Add the primelevel logic back in as check in apply.

src/treetensornetworks/solvers/contract.jl

@@ -74,6 +74,11 @@ end
 Overload of `ITensors.apply`.
 """
 function apply(tn1::AbstractTTN, tn2::AbstractTTN; init, kwargs...)
+  if !isone(plev_diff(flatten_external_indsnetwork(tn1, tn2), external_indsnetwork(init)))
+    error(
+      "Initial guess `init` needs to primelevel one less than the contraction tn1 and tn2."
+    )
+  end
   init = init'
   tn12 = contract(tn1, tn2; init, kwargs...)
   return replaceprime(tn12, 1 => 0)
@@ -82,8 +87,25 @@ end
 function sum_apply(
   tns::Vector{<:Tuple{<:AbstractTTN,<:AbstractTTN}}; alg="fit", init, kwargs...
 )
+  if !isone(
+    plev_diff(
+      flatten_external_indsnetwork(first(first(tns)), last(first(tns))),
+      external_indsnetwork(init),
+    ),
+  )
+    error(
+      "Initial guess `init` needs to primelevel one less than the contraction tn1 and tn2."
+    )
+  end
+
   init = init'
   alg != "fit" && error("sum_apply not implemented for other algorithms than fit.")
   tn12 = contract(Algorithm(alg), tns; init, kwargs...)
   return replaceprime(tn12, 1 => 0)
 end
+
+function plev_diff(a::IndsNetwork, b::IndsNetwork)
+  pla = plev(only(a[first(vertices(a))]))
+  plb = plev(only(b[first(vertices(b))]))
+  return pla - plb
+end