diff --git a/docs/src/getting_started.md b/docs/src/getting_started.md
index 24f9b95cc..d1a75be20 100644
--- a/docs/src/getting_started.md
+++ b/docs/src/getting_started.md
@@ -20,7 +20,7 @@ using Symbolics
 ```
 
 After defining variables as symbolic, symbolic expressions, which we call a
-`istree` object, can be generated by utilizing Julia expressions. For example:
+`iscall` object, can be generated by utilizing Julia expressions. For example:
 
 ```@example symbolic_basics
 z = x^2 + y
@@ -35,7 +35,7 @@ A = [x^2 + y 0 2x
      y^2 + x 0 0]
 ```
 
-Note that by default, `@variables` returns `Sym` or `istree` objects wrapped in
+Note that by default, `@variables` returns `Sym` or `iscall` objects wrapped in
 `Num` to make them behave like subtypes of `Real`. Any operation on
 these `Num` objects will return a new `Num` object, wrapping the result of
 computing symbolically on the underlying values.
diff --git a/docs/src/manual/variables.md b/docs/src/manual/variables.md
index ee05c74d4..97d9b7266 100644
--- a/docs/src/manual/variables.md
+++ b/docs/src/manual/variables.md
@@ -3,7 +3,7 @@
 Symbolics IR mirrors the Julia AST but allows for easy mathematical
 manipulation by itself following mathematical semantics. The base of the IR is
 the `Sym` type, which defines a symbolic variable. Registered (mathematical)
-functions on `Sym`s (or `istree` objects) return an expression that `istree`.
+functions on `Sym`s (or `iscall` objects) return an expression that `iscall`.
 For example, `op1 = x+y` is one symbolic object and `op2 = 2z` is another, and
 so `op1*op2` is another tree object. Then, at the top, an `Equation`, normally
 written as `op1 ~ op2`, defines the symbolic equality between two operations.
@@ -13,10 +13,10 @@ written as `op1 ~ op2`, defines the symbolic equality between two operations.
 `Sym`, `Term`, and `FnType` are from
 [SymbolicUtils.jl](https://symbolicutils.juliasymbolics.org/api/). Note that in
 Symbolics, we always use `Sym{Real}`, `Term{Real}`, and
-`FnType{Tuple{Any}, Real}`. To get the arguments of an `istree` object, use
+`FnType{Tuple{Any}, Real}`. To get the arguments of an `iscall` object, use
 `arguments(t::Term)`, and to get the operation, use `operation(t::Term)`.
 However, note that one should never dispatch on `Term` or test `isa Term`.
-Instead, one needs to use `SymbolicUtils.istree` to check if `arguments` and
+Instead, one needs to use `SymbolicUtils.iscall` to check if `arguments` and
 `operation` is defined.
 
 ```@docs
@@ -80,7 +80,7 @@ Control flow can be expressed in Symbolics.jl in the following ways:
 ## Inspection Functions
 
 ```@docs
-SymbolicUtils.istree
+SymbolicUtils.iscall
 SymbolicUtils.operation
 SymbolicUtils.arguments
 ```
diff --git a/ext/SymbolicsSymPyExt.jl b/ext/SymbolicsSymPyExt.jl
index 630cd3ce7..9a92e06d7 100644
--- a/ext/SymbolicsSymPyExt.jl
+++ b/ext/SymbolicsSymPyExt.jl
@@ -9,13 +9,13 @@ end
 
 using Symbolics: value
 using Symbolics.SymbolicUtils
-using SymbolicUtils: istree, operation, arguments, symtype,
+using SymbolicUtils: iscall, operation, arguments, symtype,
                         FnType, Symbolic
 
 function Symbolics.symbolics_to_sympy(expr)
     expr = value(expr)
     expr isa Symbolic || return expr
-    if istree(expr)
+    if iscall(expr)
         sop = symbolics_to_sympy(operation(expr))
         sargs = map(symbolics_to_sympy, arguments(expr))
         if sop === (^) && length(sargs) == 2 && sargs[2] isa Number
diff --git a/src/Symbolics.jl b/src/Symbolics.jl
index b48673c71..8cf10baa3 100644
--- a/src/Symbolics.jl
+++ b/src/Symbolics.jl
@@ -19,7 +19,7 @@ using PrecompileTools
 
     import DomainSets: Domain
 
-    import SymbolicUtils: similarterm, istree, operation, arguments, symtype, metadata
+    import SymbolicUtils: similarterm, iscall, operation, arguments, symtype, metadata
 
     import SymbolicUtils: Term, Add, Mul, Pow, Sym, Div, BasicSymbolic,
     FnType, @rule, Rewriters, substitute,
diff --git a/src/array-lib.jl b/src/array-lib.jl
index cc5923032..242d8eff8 100644
--- a/src/array-lib.jl
+++ b/src/array-lib.jl
@@ -20,7 +20,7 @@ end
 function Base.getindex(x::SymArray, idx...)
     idx = unwrap.(idx)
     meta = metadata(unwrap(x))
-    if istree(x) && (op = operation(x)) isa Operator
+    if iscall(x) && (op = operation(x)) isa Operator
         args = arguments(x)
         return op(only(args)[idx...])
     elseif shape(x) !== Unknown() && all(i -> i isa Integer, idx)
@@ -111,7 +111,7 @@ end
 
 import Base: +, -, *
 tup(c::CartesianIndex) = Tuple(c)
-tup(c::Symbolic{CartesianIndex}) = istree(c) ? arguments(c) : error("Cartesian index not found")
+tup(c::Symbolic{CartesianIndex}) = iscall(c) ? arguments(c) : error("Cartesian index not found")
 
 @wrapped function -(x::CartesianIndex, y::CartesianIndex)
     CartesianIndex((tup(x) .- tup(y))...)
@@ -251,7 +251,7 @@ isadjointvec(A::Adjoint) = ndims(parent(A)) == 1
 isadjointvec(A::Transpose) = ndims(parent(A)) == 1
 
 function isadjointvec(A)
-    if istree(A)
+    if iscall(A)
         (operation(A) === (adjoint) ||
          operation(A) == (transpose)) && ndims(arguments(A)[1]) == 1
     else
@@ -305,7 +305,7 @@ function _matvec(A, b)
 end
 @wrapped (*)(A::AbstractMatrix, b::AbstractVector) = _matvec(A, b)
 
-# specialize `dot` to dispatch on `Symbolic{<:Number}` to eventually work for 
+# specialize `dot` to dispatch on `Symbolic{<:Number}` to eventually work for
 # arrays of (possibly unwrapped) Symbolic types, see issue #831
 @wrapped LinearAlgebra.dot(x::Number, y::Number) = conj(x) * y
 
diff --git a/src/arrays.jl b/src/arrays.jl
index 3b2f069f2..242cb05fe 100644
--- a/src/arrays.jl
+++ b/src/arrays.jl
@@ -94,7 +94,7 @@ shape(aop::ArrayOp) = aop.shape
 
 const show_arrayop = Ref{Bool}(false)
 function Base.show(io::IO, aop::ArrayOp)
-    if istree(aop.term) && !show_arrayop[]
+    if iscall(aop.term) && !show_arrayop[]
         show(io, aop.term)
     else
         print(io, "@arrayop")
@@ -117,7 +117,7 @@ function Base.showarg(io::IO, aop::ArrayOp, toplevel)
 end
 
 symtype(a::ArrayOp{T}) where {T} = T
-istree(a::ArrayOp) = true
+iscall(a::ArrayOp) = true
 function operation(a::ArrayOp)
     isnothing(a.term) ? typeof(a) : operation(a.term)
 end
@@ -332,7 +332,7 @@ function get_extents(xs)
     if all(iszero∘wrap, boundaries)
         get(first(xs))
     else
-        ii = findfirst(x->issym(x) || istree(x), boundaries)
+        ii = findfirst(x->issym(x) || iscall(x), boundaries)
         if !isnothing(ii)
             error("Could not find the boundary from symbolic index $(xs[ii]). Please manually specify the range of indices.")
         end
@@ -355,11 +355,11 @@ get_extents(x::AbstractRange) = x
 # boundary: how much padding is this indexing requiring, for example
 #   boundary is 2 for x[i + 2], and boundary = -2 for x[i - 2]
 function idx_to_axes(expr, dict=Dict{Any, Vector}(), ranges=Dict())
-    if istree(expr)
+    if iscall(expr)
         if operation(expr) === (getindex)
             args = arguments(expr)
             for (axis, idx_expr) in enumerate(@views args[2:end])
-                if issym(idx_expr) || istree(idx_expr)
+                if issym(idx_expr) || iscall(idx_expr)
                     vs = get_variables(idx_expr)
                     isempty(vs) && continue
                     sym = only(get_variables(idx_expr))
@@ -529,9 +529,9 @@ wrapper_type(::Type{<:AbstractVector{T}}) where {T} = Arr{maybewrap(T), 1}
 
 function Base.show(io::IO, arr::Arr)
     x = unwrap(arr)
-    istree(x) && print(io, "(")
+    iscall(x) && print(io, "(")
     print(io, unwrap(arr))
-    istree(x) && print(io, ")")
+    iscall(x) && print(io, ")")
     if !(shape(x) isa Unknown)
         print(io, "[", join(string.(axes(arr)), ","), "]")
     end
@@ -618,7 +618,7 @@ function replace_by_scalarizing(ex, dict)
     end
 
     function rewrite_operation(x)
-        if istree(x) && istree(operation(x))
+        if iscall(x) && iscall(operation(x))
             f = operation(x)
             ff = replace_by_scalarizing(f, dict)
             if metadata(x) !== nothing
@@ -638,7 +638,7 @@ end
 
 function prewalk_if(cond, f, t, similarterm)
     t′ = cond(t) ? f(t) : return t
-    if istree(t′)
+    if iscall(t′)
         return similarterm(t′, operation(t′),
                            map(x->prewalk_if(cond, f, x, similarterm), arguments(t′)))
     else
@@ -652,7 +652,7 @@ function scalarize(arr::AbstractArray, idx)
 end
 
 function scalarize(arr, idx)
-    if istree(arr)
+    if iscall(arr)
         scalarize_op(operation(arr), arr, idx)
     else
         error("scalarize is not defined for $arr at idx=$idx")
@@ -761,20 +761,20 @@ eval_array_term(op) = eval_array_term(operation(op), op)
 
 function scalarize(arr)
     if arr isa Arr || arr isa Symbolic{<:AbstractArray}
-        if istree(arr)
+        if iscall(arr)
             arr = eval_array_term(arr)
         end
         map(Iterators.product(axes(arr)...)) do i
             scalarize(arr[i...]) # Use arr[i...] here to trigger any getindex hooks
         end
-    elseif istree(arr) && operation(arr) == getindex
+    elseif iscall(arr) && operation(arr) == getindex
         args = arguments(arr)
         scalarize(args[1], (args[2:end]...,))
     elseif arr isa Num
         wrap(scalarize(unwrap(arr)))
-    elseif istree(arr) && symtype(arr) <: Number
+    elseif iscall(arr) && symtype(arr) <: Number
         t = similarterm(arr, operation(arr), map(scalarize, arguments(arr)), symtype(arr), metadata=metadata(arr))
-        istree(t) ? scalarize_op(operation(t), t) : t
+        iscall(t) ? scalarize_op(operation(t), t) : t
     else
         arr
     end
@@ -804,7 +804,7 @@ function arraymaker(T, shape, views, seq...)
     ArrayMaker{T}(shape, [(views .=> seq)...], nothing)
 end
 
-istree(x::ArrayMaker) = true
+iscall(x::ArrayMaker) = true
 operation(x::ArrayMaker) = arraymaker
 arguments(x::ArrayMaker) = [eltype(x), shape(x), map(first, x.sequence), map(last, x.sequence)...]
 
@@ -965,7 +965,7 @@ end
 
 function scalarize(x::ArrayMaker, idx)
     for (vw, arr) in reverse(x.sequence) # last one wins
-        if any(x->issym(x) || istree(x), idx)
+        if any(x->issym(x) || iscall(x), idx)
             return term(getindex, x, idx...)
         end
         if all(in.(idx, vw))
@@ -979,7 +979,7 @@ function scalarize(x::ArrayMaker, idx)
             end
         end
     end
-    if !any(x->issym(x) || istree(x), idx) && all(in.(idx, axes(x)))
+    if !any(x->issym(x) || iscall(x), idx) && all(in.(idx, axes(x)))
         throw(UndefRefError())
     end
 
@@ -992,7 +992,7 @@ end
 function SymbolicUtils.Code.toexpr(x::ArrayOp, st)
     haskey(st.symbolify, x) && return st.symbolify[x]
 
-    if istree(x.term)
+    if iscall(x.term)
         toexpr(x.term, st)
     else
         _array_toexpr(x, st)
@@ -1048,7 +1048,7 @@ end
 
 function inplace_builtin(term, outsym)
     isarr(n) = x->symtype(x) <: AbstractArray{<:Any, n}
-    if istree(term) && operation(term) == (*) && length(arguments(term)) == 2
+    if iscall(term) && operation(term) == (*) && length(arguments(term)) == 2
         A, B = arguments(term)
         isarr(2)(A) && (isarr(1)(B) || isarr(2)(B)) && return :($mul!($outsym, $A, $B))
     end
@@ -1058,7 +1058,7 @@ end
 function find_inter(acc, expr)
     if !issym(expr) && symtype(expr) <: AbstractArray
         push!(acc, expr)
-    elseif istree(expr)
+    elseif iscall(expr)
         foreach(x -> find_inter(acc, x), arguments(expr))
     end
     acc
diff --git a/src/build_function.jl b/src/build_function.jl
index 4c0b4db8d..b0fd5f288 100644
--- a/src/build_function.jl
+++ b/src/build_function.jl
@@ -160,7 +160,7 @@ function _build_function(target::JuliaTarget, op::Union{Arr, ArrayOp}, args...;
 
     N = length(shape(op))
     op = unwrap(op)
-    if op isa ArrayOp && istree(op.term)
+    if op isa ArrayOp && iscall(op.term)
         op_body = op.term
     else
         op_body = :(let $outsym = zeros(Float64, map(length, ($(shape(op)...),)))
@@ -221,7 +221,7 @@ Build function target: `JuliaTarget`
 
 ```julia
 function _build_function(target::JuliaTarget, rhss, args...;
-                         conv = toexpr, 
+                         conv = toexpr,
                          expression = Val{true},
                          checkbounds = false,
                          linenumbers = false,
@@ -584,13 +584,13 @@ function numbered_expr(O::Symbolic,varnumbercache,args...;varordering = args[1],
                        states = LazyState(),
                        lhsname=:du,rhsnames=[Symbol("MTK$i") for i in 1:length(args)])
     O = value(O)
-    if (issym(O) || issym(operation(O))) || (istree(O) && operation(O) == getindex)
+    if (issym(O) || issym(operation(O))) || (iscall(O) && operation(O) == getindex)
         (j,i) = get(varnumbercache, O, (nothing, nothing))
         if !isnothing(j)
             return i==0 ? :($(rhsnames[j])) : :($(rhsnames[j])[$(i+offset)])
         end
     end
-    if istree(O)
+    if iscall(O)
         if operation(O) === getindex
             args = arguments(O)
             Expr(:ref, toexpr(args[1], states), toexpr.(args[2:end] .+ offset, (states,))...)
diff --git a/src/complex.jl b/src/complex.jl
index d1e7b4769..6fbf37a28 100644
--- a/src/complex.jl
+++ b/src/complex.jl
@@ -18,7 +18,7 @@ function wrapper_type(::Type{Complex{T}}) where T
 end
 
 symtype(a::ComplexTerm{T}) where T = Complex{T}
-istree(a::ComplexTerm) = true
+iscall(a::ComplexTerm) = true
 operation(a::ComplexTerm{T}) where T = Complex{T}
 arguments(a::ComplexTerm) = [a.re, a.im]
 metadata(a::ComplexTerm) = a.re.metadata
@@ -41,8 +41,8 @@ function Base.show(io::IO, a::Complex{Num})
     rr = unwrap(real(a))
     ii = unwrap(imag(a))
 
-    if istree(rr) && (operation(rr) === real) &&
-        istree(ii) && (operation(ii) === imag) &&
+    if iscall(rr) && (operation(rr) === real) &&
+        iscall(ii) && (operation(ii) === imag) &&
         isequal(arguments(rr)[1], arguments(ii)[1])
 
         return print(io, arguments(rr)[1])
diff --git a/src/diff.jl b/src/diff.jl
index 7e91d19d8..aeb2cc78e 100644
--- a/src/diff.jl
+++ b/src/diff.jl
@@ -46,7 +46,7 @@ end
 (D::Differential)(x::Complex{Num}) = wrap(ComplexTerm{Real}(D(unwrap(real(x))), D(unwrap(imag(x)))))
 SymbolicUtils.promote_symtype(::Differential, T) = T
 
-is_derivative(x) = istree(x) ? operation(x) isa Differential : false
+is_derivative(x) = iscall(x) ? operation(x) isa Differential : false
 
 Base.:*(D1, D2::Differential) = D1 ∘ D2
 Base.:*(D1::Differential, D2) = D1 ∘ D2
@@ -59,7 +59,7 @@ Base.:(==)(D1::Differential, D2::Differential) = isequal(D1.x, D2.x)
 Base.hash(D::Differential, u::UInt) = hash(D.x, xor(u, 0xdddddddddddddddd))
 
 _isfalse(occ::Bool) = occ === false
-_isfalse(occ::Symbolic) = istree(occ) && _isfalse(operation(occ))
+_isfalse(occ::Symbolic) = iscall(occ) && _isfalse(operation(occ))
 
 function occursin_info(x, expr, fail = true)
     if symtype(expr) <: AbstractArray
@@ -72,8 +72,8 @@ function occursin_info(x, expr, fail = true)
 
     # Allow scalarized expressions
     function is_scalar_indexed(ex)
-        (istree(ex) && operation(ex) == getindex && !(symtype(ex) <: AbstractArray)) ||
-        (istree(ex) && (issym(operation(ex)) || istree(operation(ex))) &&
+        (iscall(ex) && operation(ex) == getindex && !(symtype(ex) <: AbstractArray)) ||
+        (iscall(ex) && (issym(operation(ex)) || iscall(operation(ex))) &&
          is_scalar_indexed(operation(ex)))
     end
 
@@ -93,7 +93,7 @@ function occursin_info(x, expr, fail = true)
         return false
     end
 
-    !istree(expr) && return isequal(x, expr)
+    !iscall(expr) && return isequal(x, expr)
     if isequal(x, expr)
         true
     else
@@ -132,7 +132,7 @@ An internal function that contains the logic for [`hasderiv`](@ref) and [`hasdif
 Return true if `O` contains a term with `Operator` `op`.
 """
 function recursive_hasoperator(op, O)
-    istree(O) || return false
+    iscall(O) || return false
     if operation(O) isa op
         return true
     else
@@ -154,14 +154,14 @@ $(SIGNATURES)
 Expands derivatives within a symbolic expression `O`.
 
 This function recursively traverses a symbolic expression, applying the chain rule
-and other derivative rules to expand any derivatives it encounters. 
+and other derivative rules to expand any derivatives it encounters.
 
 # Arguments
 - `O::Symbolic`: The symbolic expression to expand.
-- `simplify::Bool=false`: Whether to simplify the resulting expression using 
+- `simplify::Bool=false`: Whether to simplify the resulting expression using
     [`SymbolicUtils.simplify`](@ref).
-- `occurrences=nothing`: Information about the occurrences of the independent 
-    variable in the argument of the derivative. This is used internally for 
+- `occurrences=nothing`: Information about the occurrences of the independent
+    variable in the argument of the derivative. This is used internally for
     optimization purposes.
 
 # Examples
@@ -179,7 +179,7 @@ julia> dfx=expand_derivatives(Dx(f))
 ```
 """
 function expand_derivatives(O::Symbolic, simplify=false; occurrences=nothing)
-    if istree(O) && isa(operation(O), Differential)
+    if iscall(O) && isa(operation(O), Differential)
         arg = only(arguments(O))
         arg = expand_derivatives(arg, false)
 
@@ -192,7 +192,7 @@ function expand_derivatives(O::Symbolic, simplify=false; occurrences=nothing)
 
         D = operation(O)
 
-        if !istree(arg)
+        if !iscall(arg)
             return D(arg) # Cannot expand
         elseif (op = operation(arg); issym(op))
             inner_args = arguments(arg)
@@ -218,7 +218,7 @@ function expand_derivatives(O::Symbolic, simplify=false; occurrences=nothing)
             else
                 inner = expand_derivatives(D(arguments(arg)[1]), false)
                 # if the inner expression is not expandable either, return
-                if istree(inner) && operation(inner) isa Differential
+                if iscall(inner) && operation(inner) isa Differential
                     return D(arg)
                 else
                     return expand_derivatives(op(inner), simplify)
@@ -230,12 +230,12 @@ function expand_derivatives(O::Symbolic, simplify=false; occurrences=nothing)
                 a, b = DomainSets.endpoints(domain)
                 c = 0
                 inner_function = expand_derivatives(arguments(arg)[1])
-                if istree(value(a))
+                if iscall(value(a))
                     t1 = SymbolicUtils.substitute(inner_function, Dict(op.domain.variables => value(a)))
                     t2 = D(a)
                     c -= t1*t2
                 end
-                if istree(value(b))
+                if iscall(value(b))
                     t1 = SymbolicUtils.substitute(inner_function, Dict(op.domain.variables => value(b)))
                     t2 = D(b)
                     c += t1*t2
@@ -283,7 +283,7 @@ function expand_derivatives(O::Symbolic, simplify=false; occurrences=nothing)
             x = +((!_iszero(c) ? vcat(c, exprs) : exprs)...)
             return simplify ? SymbolicUtils.simplify(x) : x
         end
-    elseif istree(O) && isa(operation(O), Integral)
+    elseif iscall(O) && isa(operation(O), Integral)
         return operation(O)(expand_derivatives(arguments(O)[1]))
     elseif !hasderiv(O)
         return O
@@ -342,7 +342,7 @@ sin(x)
 """
 derivative_idx(O::Any, ::Any) = 0
 function derivative_idx(O::Symbolic, idx)
-    istree(O) ? derivative(operation(O), (arguments(O)...,), Val(idx)) : 0
+    iscall(O) ? derivative(operation(O), (arguments(O)...,), Val(idx)) : 0
 end
 
 # Indicate that no derivative is defined.
diff --git a/src/extra_functions.jl b/src/extra_functions.jl
index 040f14f72..53fc0d2f7 100644
--- a/src/extra_functions.jl
+++ b/src/extra_functions.jl
@@ -4,7 +4,7 @@ function _binomial(nothing, n, k)
         args = [n, k]
         unwrapped_args = map(Symbolics.unwrap, args)
         res = if !(any((x->begin
-                                SymbolicUtils.issym(x) || SymbolicUtils.istree(x)
+                                SymbolicUtils.issym(x) || SymbolicUtils.iscall(x)
                             end), unwrapped_args))
                 Base.binomial(unwrapped_args...)
             else
diff --git a/src/latexify_recipes.jl b/src/latexify_recipes.jl
index 8fb2b578b..3174dff8f 100644
--- a/src/latexify_recipes.jl
+++ b/src/latexify_recipes.jl
@@ -3,7 +3,7 @@ prettify_expr(f::Function) = nameof(f)
 prettify_expr(expr::Expr) = Expr(expr.head, prettify_expr.(expr.args)...)
 
 function cleanup_exprs(ex)
-    return postwalk(x -> istree(x) && length(arguments(x)) == 0 ? operation(x) : x, ex)
+    return postwalk(x -> iscall(x) && length(arguments(x)) == 0 ? operation(x) : x, ex)
 end
 
 function latexify_derivatives(ex)
@@ -58,7 +58,7 @@ end
 @latexrecipe function f(z::Complex{Num})
     env --> :equation
     cdot --> false
-    
+
     iszero(z.im) && return :($(recipe(z.re)))
     iszero(z.re) && return :($(recipe(z.im)) * $im)
     return :($(recipe(z.re)) + $(recipe(z.im)) * $im)
@@ -142,7 +142,7 @@ function _toexpr(O)
 
             base = term.base
             pow  = term.exp
-            isneg = (pow isa Number && pow < 0) || (istree(pow) && operation(pow) === (-) && length(arguments(pow)) == 1)
+            isneg = (pow isa Number && pow < 0) || (iscall(pow) && operation(pow) === (-) && length(arguments(pow)) == 1)
             if !isneg
                 if _isone(pow)
                     pushfirst!(numer, _toexpr(base))
@@ -179,7 +179,7 @@ function _toexpr(O)
         end
     end
     issym(O) && return nameof(O)
-    !istree(O) && return O
+    !iscall(O) && return O
 
     op = operation(O)
     args = arguments(O)
@@ -233,7 +233,7 @@ _toexpr(eqs::AbstractArray) = map(eq->_toexpr(eq), eqs)
 _toexpr(x::Num) = _toexpr(value(x))
 
 function getindex_to_symbol(t)
-    @assert istree(t) && operation(t) === getindex && symtype(arguments(t)[1]) <: AbstractArray
+    @assert iscall(t) && operation(t) === getindex && symtype(arguments(t)[1]) <: AbstractArray
     args = arguments(t)
     idxs = args[2:end]
     try
@@ -258,4 +258,3 @@ function diffdenom(e)
         e
     end
 end
-
diff --git a/src/linear_algebra.jl b/src/linear_algebra.jl
index f13bc8854..a3d9a706f 100644
--- a/src/linear_algebra.jl
+++ b/src/linear_algebra.jl
@@ -1,5 +1,5 @@
 function nterms(t)
-    if istree(t)
+    if iscall(t)
         return reduce(+, map(nterms, arguments(t)), init=0)
     else
         return 1
@@ -258,7 +258,7 @@ function _linear_expansion(t::Equation, x)
 end
 trivial_linear_expansion(t, x) = isequal(t, x) ? (1, 0, true) : (0, t, true)
 
-is_expansion_leaf(t) = !istree(t) || (operation(t) isa Operator)
+is_expansion_leaf(t) = !iscall(t) || (operation(t) isa Operator)
 @noinline expansion_check(op) = op isa Operator && error("The operation is an Operator. This should never happen.")
 function _linear_expansion(t, x)
     t = value(t)
diff --git a/src/register.jl b/src/register.jl
index 51c4152ac..05ddf0d7e 100644
--- a/src/register.jl
+++ b/src/register.jl
@@ -32,7 +32,7 @@ macro register_symbolic(expr, define_promotion = true, Ts = :([]))
     fexpr = :(Symbolics.@wrapped function $f($(args′...))
                   args = [$(argnames...),]
                   unwrapped_args = map($unwrap, args)
-                  res = if !any(x->$issym(x) || $istree(x), unwrapped_args)
+                  res = if !any(x->$issym(x) || $iscall(x), unwrapped_args)
                       $f(unwrapped_args...) # partial-eval if all args are unwrapped
                   else
                       $Term{$ret_type}($f, unwrapped_args)
@@ -94,7 +94,7 @@ function register_array_symbolic(f, ftype, argnames, Ts, ret_type, partial_defs
         @wrapped function $f($(args′...))
             args = [$(argnames...),]
             unwrapped_args = map($unwrap, args)
-            res = if !any(x->$issym(x) || $istree(x), unwrapped_args)
+            res = if !any(x->$issym(x) || $iscall(x), unwrapped_args)
                 $f(unwrapped_args...) # partial-eval if all args are unwrapped
             elseif $ret_type == nothing || ($ret_type <: AbstractArray)
                 $array_term($(Expr(:parameters, [Expr(:kw, k, v) for (k, v) in defs]...)), $f, unwrapped_args...)
diff --git a/src/rewrite-helpers.jl b/src/rewrite-helpers.jl
index b2a7d0249..cfb917d50 100644
--- a/src/rewrite-helpers.jl
+++ b/src/rewrite-helpers.jl
@@ -73,7 +73,7 @@ function _occursin(r, y)
         end
     end
 
-    if istree(y)
+    if iscall(y)
         return r(operation(y)) ||
                 any(y->_occursin(r, y), arguments(y))
     else
@@ -116,7 +116,7 @@ function filterchildren!(r::Any, y, acc)
         end
     end
 
-    if istree(y)
+    if iscall(y)
         if isequal(r, operation(y))
             push!(acc, operation(y))
         elseif r isa Function && r(operation(y))
diff --git a/src/semipoly.jl b/src/semipoly.jl
index 2593ae4ae..46b635ba3 100644
--- a/src/semipoly.jl
+++ b/src/semipoly.jl
@@ -23,7 +23,7 @@ function Base.:+(a::SemiMonomial, b::SemiMonomial)
     Term(+, [a, b])
 end
 function Base.:+(m::SemiMonomial, t)
-    if istree(t) && operation(t) == (+)
+    if iscall(t) && operation(t) == (+)
         return Term(+, [unsorted_arguments(t); m])
     end
     Term(+, [m, t])
@@ -36,7 +36,7 @@ function Base.:*(a::SemiMonomial, b::SemiMonomial)
 end
 Base.:*(m::SemiMonomial, n::Number) = SemiMonomial(m.p, m.coeff * n)
 function Base.:*(m::SemiMonomial, t::Symbolic)
-    if istree(t)
+    if iscall(t)
         op = operation(t)
         if op == (+)
             args = collect(all_terms(t))
@@ -70,7 +70,7 @@ function pdegrees(x)
         dict = pdegrees(x.base)
         degrees = map(degree -> degree * x.exp, values(dict))
         Dict(keys(dict) .=> degrees)
-    elseif issym(x) || istree(x)
+    elseif issym(x) || iscall(x)
         return Dict(x=>1)
     elseif x isa Number
         return Dict()
@@ -133,7 +133,7 @@ issym(::SemiMonomial) = true
 
 Base.:nameof(m::SemiMonomial) = Symbol(:SemiMonomial, m.p, m.coeff)
 
-isop(x, op) = istree(x) && operation(x) === op
+isop(x, op) = iscall(x) && operation(x) === op
 isop(op) = Base.Fix2(isop, op)
 
 bareterm(x, f, args; kw...) = Term{symtype(x)}(f, args)
@@ -158,7 +158,7 @@ end
 
 function semipolyform_terms(expr, vars)
     expr = mark_and_exponentiate(expr, vars)
-    if istree(expr) && operation(expr) == (+)
+    if iscall(expr) && operation(expr) == (+)
         args = collect(all_terms(expr))
         return args
     elseif isreal(expr) && iszero(real(expr)) # when `expr` is just a 0
@@ -177,7 +177,7 @@ Return true if `expr` contains any variables in `vars`.
 function has_vars(expr, vars)::Bool
     if expr in vars
         return true
-    elseif istree(expr)
+    elseif iscall(expr)
         for arg in unsorted_arguments(expr)
             if has_vars(arg, vars)
                 return true
@@ -190,7 +190,7 @@ end
 function mark_vars(expr, vars)
     if expr in vars
         return SemiMonomial(expr, 1)
-    elseif !istree(expr)
+    elseif !iscall(expr)
         return SemiMonomial(1, expr)
     end
     op = operation(expr)
@@ -403,7 +403,7 @@ end
 
 ## Utilities
 
-all_terms(x) = istree(x) && operation(x) == (+) ? collect(Iterators.flatten(map(all_terms, unsorted_arguments(x)))) : (x,)
+all_terms(x) = iscall(x) && operation(x) == (+) ? collect(Iterators.flatten(map(all_terms, unsorted_arguments(x)))) : (x,)
 
 function unwrap_sp(m::SemiMonomial)
     degree_dict = pdegrees(m.p)
@@ -424,7 +424,7 @@ function unwrap_sp(m::SemiMonomial)
 end
 function unwrap_sp(x)
     x = unwrap(x)
-    istree(x) ? similarterm(x, operation(x), map(unwrap_sp, unsorted_arguments(x))) : x
+    iscall(x) ? similarterm(x, operation(x), map(unwrap_sp, unsorted_arguments(x))) : x
 end
 
 function cautious_sum(nls)
diff --git a/src/solver.jl b/src/solver.jl
index 020c3e58b..420670083 100644
--- a/src/solver.jl
+++ b/src/solver.jl
@@ -81,7 +81,7 @@ function get_parts_list(a, b, a_list = Vector{Any}(), b_list = Vector{Any}())
     if SymbolicUtils.issym(a)
         push!(a_list, a)
         push!(b_list, b)
-    elseif istree(a) && istree(b) && isequal(operation(a), operation(b))
+    elseif iscall(a) && iscall(b) && isequal(operation(a), operation(b))
         a_args = arguments(a)
         b_args = arguments(b)
 
@@ -160,7 +160,7 @@ end
 function replace_term(expr, dic::Dict)
     if SymbolicUtils.issym(expr) && haskey(dic, expr)
         return dic[expr]
-    elseif istree(expr)
+    elseif iscall(expr)
         args = Any[]
 
         for arg in arguments(expr)
@@ -202,9 +202,9 @@ end
 function expr_similar(ref_expr, expr, check_matches = true)
 
     SymbolicUtils.issym(ref_expr) && return true
-    SymbolicUtils.issym(expr) && istree(ref_expr) && return false
+    SymbolicUtils.issym(expr) && iscall(ref_expr) && return false
 
-    if istree(ref_expr)
+    if iscall(ref_expr)
         ref_args = arguments(ref_expr)
         ref_len = length(ref_args)
         ref_op = operation(ref_expr)
@@ -249,12 +249,12 @@ function expr_similar(ref_expr, expr, check_matches = true)
 end
 
 function get_base(expr)
-    (!istree(expr) || operation(expr) != (^)) && throw("not a power(^) -> $expr")
+    (!iscall(expr) || operation(expr) != (^)) && throw("not a power(^) -> $expr")
     return arguments(expr)[1]
 end
 
 function get_exp(expr)
-    (!istree(expr) || operation(expr) != (^)) && throw("not a power(^) -> $expr")
+    (!iscall(expr) || operation(expr) != (^)) && throw("not a power(^) -> $expr")
     return arguments(expr)[2]
 end
 
@@ -270,7 +270,7 @@ function solve_single_eq_unchecked(
     while (true)
         oldState = eq
 
-        if (istree(eq.lhs))
+        if (iscall(eq.lhs))
 
             potential_solution = solve_quadratic(eq, var, single_solution)
             if potential_solution isa Equation
@@ -366,7 +366,7 @@ example move_to_other_side(x+a~z,x) returns x~z-a
 =#
 function move_to_other_side(eq::Equation, var)
 
-    !istree(eq.lhs) && return eq#make sure left side is tree form
+    !iscall(eq.lhs) && return eq#make sure left side is tree form
 
     op = operation(eq.lhs)
 
@@ -421,14 +421,14 @@ function special_strategy(eq::Equation, var)
     end
 
 
-    !istree(eq.lhs) && return eq#make sure left side is tree form
+    !iscall(eq.lhs) && return eq#make sure left side is tree form
 
     op = operation(eq.lhs)
     elements = arguments(eq.lhs)
 
     if (op == +) &&
        length(elements) == 2 &&
-       sum(istree.(elements)) == length(elements) &&
+       sum(iscall.(elements)) == length(elements) &&
        isequal(operation.(elements), [sqrt for el = 1:length(elements)]) #check for sqrt(a)+sqrt(b)=c form , to solve this sqrt(a)+sqrt(b)=c -> 4*a*b-full_expand((c^2-b-a)^2)=0 then solve using quadratics
 
         #grab values
@@ -445,10 +445,10 @@ function special_strategy(eq::Equation, var)
     elseif (op == +) &&
            isequal(eq.rhs, 0) &&
            length(elements) == 2 &&
-           sum(istree.(elements)) == length(elements) &&
+           sum(iscall.(elements)) == length(elements) &&
            length(arguments(elements[1])) == 2 &&
            isequal(arguments(elements[1])[1], -1) &&
-           istree(arguments(elements[1])[2]) &&
+           iscall(arguments(elements[1])[2]) &&
            operation(elements[2]) == operation(arguments(elements[1])[2])#-f(y)+f(x)=0 -> x-y=0
 
         x = arguments(elements[2])[1]
@@ -473,16 +473,16 @@ function reduce_root(a)
         a = term(^, a.base, a.exp)
     end
 
-    if istree(a) && (operation(a) == sqrt)
+    if iscall(a) && (operation(a) == sqrt)
         a = SymbolicUtils.Pow(arguments(a)[1], 1 // 2)
-    elseif istree(a) &&
+    elseif iscall(a) &&
            (operation(a) == ^) &&
            isequal(arguments(a)[2], 1 // 2) &&
            !(arguments(a)[1] isa Number)
         a = term(sqrt, arguments(a)[1])
     end
 
-    if istree(a) &&
+    if iscall(a) &&
        (operation(a) == ^) &&
        arguments(a)[2] isa Rational &&
        isequal((arguments(a)[2]).num, 1)
@@ -531,7 +531,7 @@ if in quadratic form returns solutions
 =#
 function solve_quadratic(eq::Equation, var, single_solution)
 
-    !istree(eq.lhs) && return eq#make sure left side is tree form
+    !iscall(eq.lhs) && return eq#make sure left side is tree form
 
     op = operation(eq.lhs)
 
@@ -579,7 +579,7 @@ end
 #reverse certain functions
 function inverse_funcs(eq::Equation, var)
 
-    !istree(eq.lhs) && return eq#make sure left side is tree form
+    !iscall(eq.lhs) && return eq#make sure left side is tree form
     op = operation(eq.lhs)
 
     #reverse functions
@@ -611,7 +611,7 @@ end
 
 #solves for powers
 function reverse_powers(eq::Equation, var, single_solution)
-    !istree(eq.lhs) && return eq#make sure left side is tree form
+    !iscall(eq.lhs) && return eq#make sure left side is tree form
     op = operation(eq.lhs)
 
     if (op == ^)
diff --git a/src/utils.jl b/src/utils.jl
index 38c861877..b6988f6ea 100644
--- a/src/utils.jl
+++ b/src/utils.jl
@@ -49,10 +49,10 @@ get_variables!(vars, e::Num, varlist=nothing) = get_variables!(vars, value(e), v
 get_variables!(vars, e, varlist=nothing) = vars
 
 function is_singleton(e)
-    if istree(e)
+    if iscall(e)
         op = operation(e)
         op === getindex && return true
-        istree(op) && return is_singleton(op) # recurse to reach getindex for array element variables
+        iscall(op) && return is_singleton(op) # recurse to reach getindex for array element variables
         return issym(op)
     else
         return issym(e)
@@ -100,7 +100,7 @@ var"z(t)[1]ˍt"
 ```
 """
 function diff2term(O, O_metadata::Union{Dict, Nothing, Base.ImmutableDict}=nothing)
-    istree(O) || return O
+    iscall(O) || return O
     if is_derivative(O)
         ds = ""
         while is_derivative(O)
@@ -119,7 +119,7 @@ function diff2term(O, O_metadata::Union{Dict, Nothing, Base.ImmutableDict}=nothi
         oldop = operation(O)
         if issym(oldop)
             opname = string(nameof(oldop))
-        elseif istree(oldop) && operation(oldop) === getindex
+        elseif iscall(oldop) && operation(oldop) === getindex
             opname = string(nameof(arguments(oldop)[1]))
             args = arguments(O)
         elseif oldop == getindex
@@ -160,7 +160,7 @@ julia> Symbolics.tosymbol(z; escape=false)
 function tosymbol(t; states=nothing, escape=true)
     if issym(t)
         return nameof(t)
-    elseif istree(t)
+    elseif iscall(t)
         if issym(operation(t))
             if states !== nothing && !(t in states)
                 return nameof(operation(t))
@@ -224,7 +224,7 @@ function var_from_nested_derivative(x,i=0)
     x = unwrap(x)
     if issym(x)
         (x, i)
-    elseif istree(x)
+    elseif iscall(x)
         operation(x) isa Differential ?
             var_from_nested_derivative(first(arguments(x)), i + 1) : (x, i)
     else
diff --git a/src/variable.jl b/src/variable.jl
index d2b461d80..8722fbb04 100644
--- a/src/variable.jl
+++ b/src/variable.jl
@@ -236,7 +236,7 @@ struct CallWithMetadata{T,M} <: Symbolic{T}
     metadata::M
 end
 
-for f in [:istree, :operation, :arguments]
+for f in [:iscall, :operation, :arguments]
     @eval SymbolicUtils.$f(x::CallWithMetadata) = $f(x.f)
 end
 
@@ -383,10 +383,10 @@ _getname(x, _) = nameof(x)
 _getname(x::Symbol, _) = x
 function _getname(x::Symbolic, val)
     issym(x) && return nameof(x)
-    if istree(x) && issym(operation(x))
+    if iscall(x) && issym(operation(x))
         return nameof(operation(x))
     end
-    if !hasmetadata(x, Symbolics.GetindexParent) && istree(x) && operation(x) == getindex
+    if !hasmetadata(x, Symbolics.GetindexParent) && iscall(x) && operation(x) == getindex
         return _getname(arguments(x)[1], val)
     end
     ss = getsource(x, nothing)
@@ -405,7 +405,7 @@ SymbolicIndexingInterface.symbolic_type(::Type{<:Symbolics.Arr}) = ArraySymbolic
 SymbolicIndexingInterface.hasname(x::Union{Num,Arr}) = hasname(unwrap(x))
 
 function SymbolicIndexingInterface.hasname(x::Symbolic)
-    issym(x) || !istree(x) || istree(x) && (issym(operation(x)) || operation(x) == getindex)
+    issym(x) || !iscall(x) || iscall(x) && (issym(operation(x)) || operation(x) == getindex)
 end
 
 SymbolicIndexingInterface.getname(x, val=_fail) = _getname(unwrap(x), val)
@@ -474,7 +474,7 @@ function fast_substitute(expr, subs; operator = Nothing)
     if (_val = get(subs, expr, nothing)) !== nothing
         return _val
     end
-    istree(expr) || return expr
+    iscall(expr) || return expr
     op = fast_substitute(operation(expr), subs; operator)
     args = SymbolicUtils.unsorted_arguments(expr)
     if !(op isa operator)
@@ -502,7 +502,7 @@ function fast_substitute(expr, pair::Pair; operator = Nothing)
             expr = fast_substitute(expr, ai => bi; operator)
         end
     end
-    istree(expr) || return expr
+    iscall(expr) || return expr
     op = fast_substitute(operation(expr), pair; operator)
     args = SymbolicUtils.unsorted_arguments(expr)
     if !(op isa operator)
@@ -528,7 +528,7 @@ function getparent(x, val=_fail)
     if maybe_parent !== nothing
         return maybe_parent
     else
-        if istree(x) && operation(x) === getindex
+        if iscall(x) && operation(x) === getindex
             return arguments(x)[1]
         end
     end
@@ -640,9 +640,9 @@ function rename(x::Symbolic, name)
         xx = @set! x.name = name
         xx = rename_metadata(x, xx, name)
         symtype(xx) <: AbstractArray ? rename_getindex_source(xx) : xx
-    elseif istree(x) && operation(x) === getindex
+    elseif iscall(x) && operation(x) === getindex
         rename(arguments(x)[1], name)[arguments(x)[2:end]...]
-    elseif istree(x) && symtype(operation(x)) <: FnType || operation(x) isa CallWithMetadata
+    elseif iscall(x) && symtype(operation(x)) <: FnType || operation(x) isa CallWithMetadata
         xx = @set x.f = rename(operation(x), name)
         @set! xx.hash = Ref{UInt}(0)
         return rename_metadata(x, xx, name)
diff --git a/test/solver.jl b/test/solver.jl
index 1a510c7f7..7a976955b 100644
--- a/test/solver.jl
+++ b/test/solver.jl
@@ -11,7 +11,7 @@ using LambertW
             return !isinteger(expr) && expr != float(pi) && expr != exp(1.0)
         elseif expr isa Equation
             return hasFloat(expr.lhs) || hasFloat(expr.rhs)
-        elseif istree(expr)
+        elseif iscall(expr)
             elements = arguments(expr)
             for element in elements
                 if hasFloat(element)