diff --git a/lib/src/atom/mod.rs b/lib/src/atom/mod.rs
index e15f0853f..9293296ef 100644
--- a/lib/src/atom/mod.rs
+++ b/lib/src/atom/mod.rs
@@ -117,7 +117,7 @@ use std::any::Any;
 use std::fmt::{Display, Debug};
 use std::convert::TryFrom;
 
-use crate::common::collections::ImmutableString;
+use crate::common::collections::{ImmutableString, CowArray};
 
 // Symbol atom
 
@@ -152,14 +152,14 @@ impl Display for SymbolAtom {
 /// An expression atom structure.
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct ExpressionAtom {
-    children: Vec<Atom>,
+    children: CowArray<Atom>,
 }
 
 impl ExpressionAtom {
     /// Constructs new expression from vector of sub-atoms. Not intended to be
     /// used directly, use [Atom::expr] instead.
     #[doc(hidden)]
-    pub(crate) fn new(children: Vec<Atom>) -> Self {
+    pub(crate) fn new(children: CowArray<Atom>) -> Self {
         Self{ children }
     }
 
@@ -170,17 +170,17 @@ impl ExpressionAtom {
 
     /// Returns a reference to a vector of sub-atoms.
     pub fn children(&self) -> &[Atom] {
-        &self.children
+        self.children.as_slice()
     }
 
     /// Returns a mutable reference to a vector of sub-atoms.
     pub fn children_mut(&mut self) -> &mut [Atom] {
-        &mut self.children
+        self.children.as_slice_mut()
     }
 
     /// Converts into a vector of sub-atoms.
     pub fn into_children(self) -> Vec<Atom> {
-        self.children
+        self.children.into()
     }
 }
 
@@ -837,7 +837,7 @@ impl Atom {
     /// assert_eq!(expr, same_expr);
     /// assert_ne!(expr, other_expr);
     /// ```
-    pub fn expr<T: Into<Vec<Atom>>>(children: T) -> Self {
+    pub fn expr<T: Into<CowArray<Atom>>>(children: T) -> Self {
         Self::Expression(ExpressionAtom::new(children.into()))
     }
 
@@ -1093,8 +1093,8 @@ mod test {
     }
 
     #[inline]
-    fn expression(children: Vec<Atom>) -> Atom {
-        Atom::Expression(ExpressionAtom{ children })
+    fn expression<const N: usize>(children: [Atom; N]) -> Atom {
+        Atom::Expression(ExpressionAtom::new(CowArray::Allocated(Box::new(children))))
     }
 
     #[inline]
@@ -1175,15 +1175,15 @@ mod test {
     #[test]
     fn test_expr_expression() {
         assert_eq!(expr!("=" ("fact" n) ("*" n ("-" n "1"))),
-            expression(vec![symbol("="), expression(vec![symbol("fact"), variable("n")]),
-            expression(vec![symbol("*"), variable("n"),
-            expression(vec![symbol("-"), variable("n"), symbol("1") ]) ]) ]));
+            expression([symbol("="), expression([symbol("fact"), variable("n")]),
+            expression([symbol("*"), variable("n"),
+            expression([symbol("-"), variable("n"), symbol("1") ]) ]) ]));
         assert_eq!(expr!("=" n {[1, 2, 3]}),
-            expression(vec![symbol("="), variable("n"), value([1, 2, 3])]));
+            expression([symbol("="), variable("n"), value([1, 2, 3])]));
         assert_eq!(expr!("=" {6} ("fact" n)),
-            expression(vec![symbol("="), value(6), expression(vec![symbol("fact"), variable("n")])]));
+            expression([symbol("="), value(6), expression([symbol("fact"), variable("n")])]));
         assert_eq!(expr!({TestMulX(3)} {TestInteger(6)}),
-            expression(vec![grounded(TestMulX(3)), grounded(TestInteger(6))]));
+            expression([grounded(TestMulX(3)), grounded(TestInteger(6))]));
     }
 
     #[test]
@@ -1239,8 +1239,8 @@ mod test {
         assert_eq!(Atom::gnd(TestMulX(3)).clone(), grounded(TestMulX(3)));
         assert_eq!(Atom::expr([Atom::sym("="), Atom::value(6),
             Atom::expr([Atom::sym("fact"), Atom::var("n")])]).clone(),
-            expression(vec![symbol("="), value(6),
-                expression(vec![symbol("fact"), variable("n")])]));
+            expression([symbol("="), value(6),
+                expression([symbol("fact"), variable("n")])]));
     }
 
     #[test]
diff --git a/lib/src/common/collections.rs b/lib/src/common/collections.rs
index da8f318d1..b1fb834cd 100644
--- a/lib/src/common/collections.rs
+++ b/lib/src/common/collections.rs
@@ -161,6 +161,97 @@ impl From<String> for ImmutableString {
     }
 }
 
+#[derive(Debug, Clone)]
+pub enum CowArray<T: 'static> {
+    Allocated(Box<[T]>),
+    Literal(&'static [T]),
+}
+
+impl<T: 'static> CowArray<T> {
+
+    pub fn new() -> Self {
+        Self::Literal(&[])
+    }
+
+    pub fn as_slice(&self) -> &[T] {
+        match self {
+            Self::Allocated(array) => &*array,
+            Self::Literal(array) => array,
+        }
+    }
+
+    pub fn as_slice_mut(&mut self) -> &mut [T] where T: Clone {
+        match self {
+            Self::Allocated(array) => &mut *array,
+            Self::Literal(array) => {
+                *self = Self::Allocated((*array).into());
+                self.as_slice_mut()
+            }
+        }
+    }
+
+    pub fn len(&self) -> usize {
+        self.as_slice().len()
+    }
+
+    pub fn iter(&self) -> impl Iterator<Item=&T> {
+        self.as_slice().iter()
+    }
+}
+
+impl<T: PartialEq> PartialEq for CowArray<T> {
+    fn eq(&self, other: &Self) -> bool {
+        self.as_slice() == other.as_slice()
+    }
+}
+
+impl<T: Eq> Eq for CowArray<T> {}
+
+impl<T: Display> Display for CowArray<T> {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "[")
+            .and_then(|_| self.as_slice().iter().take(1).fold(Ok(()),
+                |res, atom| res.and_then(|_| write!(f, "{}", atom))))
+            .and_then(|_| self.as_slice().iter().skip(1).fold(Ok(()),
+                |res, atom| res.and_then(|_| write!(f, " {}", atom))))
+            .and_then(|_| write!(f, "]"))
+    }
+}
+
+impl<T: 'static> From<&'static [T]> for CowArray<T> {
+    fn from(a: &'static [T]) -> Self {
+        CowArray::Literal(a)
+    }
+}
+
+impl<T, const N: usize> From<[T; N]> for CowArray<T> {
+    fn from(a: [T; N]) -> Self {
+        CowArray::Allocated(Box::new(a))
+    }
+}
+
+impl<T> From<Vec<T>> for CowArray<T> {
+    fn from(v: Vec<T>) -> Self {
+        CowArray::Allocated(v.into_boxed_slice())
+    }
+}
+
+impl<T: Clone> Into<Vec<T>> for CowArray<T> {
+    fn into(self) -> Vec<T> {
+        match self {
+            Self::Allocated(array) => array.into(),
+            Self::Literal(array) => array.into(),
+        }
+    }
+}
+
+impl<'a, T> Into<&'a [T]> for &'a CowArray<T> {
+    fn into(self) -> &'a [T] {
+        self.as_slice()
+    }
+}
+
+
 #[cfg(test)]
 mod test {
     use super::*;
diff --git a/lib/src/metta/interpreter_minimal.rs b/lib/src/metta/interpreter_minimal.rs
index e5133fa58..650533e2d 100644
--- a/lib/src/metta/interpreter_minimal.rs
+++ b/lib/src/metta/interpreter_minimal.rs
@@ -7,6 +7,7 @@ use crate::space::*;
 use crate::metta::*;
 use crate::metta::types::*;
 use crate::metta::runner::stdlib_minimal::IfEqualOp;
+use crate::common::collections::CowArray;
 
 use std::fmt::{Debug, Display, Formatter};
 use std::convert::TryFrom;
@@ -679,11 +680,11 @@ fn collapse_bind_ret(stack: Rc<RefCell<Stack>>, atom: Atom, bindings: Bindings)
         let Stack{ prev: _, atom: collapse, ret: _, finished: _, vars: _ } = stack_ref;
         match atom_as_slice_mut(collapse) {
             Some([_op, Atom::Expression(finished_placeholder), _bindings]) => {
-                let mut finished = ExpressionAtom::new(Vec::new());
+                let mut finished = ExpressionAtom::new(CowArray::new());
                 std::mem::swap(&mut finished, finished_placeholder);
                 let mut finished = finished.into_children();
                 finished.push(atom_bindings_into_atom(nested, bindings));
-                std::mem::swap(&mut ExpressionAtom::new(finished), finished_placeholder);
+                std::mem::swap(&mut ExpressionAtom::new(finished.into()), finished_placeholder);
             },
             _ => panic!("Unexpected state"),
         };