diff --git a/src/int_allocator.rs b/src/int_allocator.rs
index afa9e60b..85fe58fc 100644
--- a/src/int_allocator.rs
+++ b/src/int_allocator.rs
@@ -6,9 +6,10 @@ pub struct IntAtomBuf {
     end: u32,
 }
 
-enum NodePtr {
-    Atom(IntAtomBuf),
-    Pair(u32, u32),
+#[derive(Clone, Copy)]
+pub struct IntPair {
+    first: i32,
+    rest: i32,
 }
 
 pub struct IntAllocator {
@@ -16,7 +17,14 @@ pub struct IntAllocator {
     // are immutable, so once they are created, they will stay around until the
     // program completes
     u8_vec: Vec<u8>,
-    node_vec: Vec<NodePtr>,
+
+    // storage for all pairs (positive indices)
+    pair_vec: Vec<IntPair>,
+
+    // storage for all atoms (negative indices).
+    // node index -1 refers to index 0 in this vector, -2 refers to 1 and so
+    // on.
+    atom_vec: Vec<IntAtomBuf>,
 }
 
 impl Default for IntAllocator {
@@ -29,44 +37,45 @@ impl IntAllocator {
     pub fn new() -> Self {
         let mut r = IntAllocator {
             u8_vec: Vec::new(),
-            node_vec: Vec::new(),
+            pair_vec: Vec::new(),
+            atom_vec: Vec::new(),
         };
         r.u8_vec.reserve(1024 * 1024);
+        r.atom_vec.reserve(256);
+        r.pair_vec.reserve(256);
         r.u8_vec.push(1_u8);
         // Preallocated empty list
-        r.node_vec
-            .push(NodePtr::Atom(IntAtomBuf { start: 0, end: 0 }));
+        r.atom_vec.push(IntAtomBuf { start: 0, end: 0 });
         // Preallocated 1
-        r.node_vec
-            .push(NodePtr::Atom(IntAtomBuf { start: 0, end: 1 }));
+        r.atom_vec.push(IntAtomBuf { start: 0, end: 1 });
         r
     }
 }
 
 impl Allocator for IntAllocator {
-    type Ptr = u32;
+    type Ptr = i32;
     type AtomBuf = IntAtomBuf;
 
     fn new_atom(&mut self, v: &[u8]) -> Self::Ptr {
         let start = self.u8_vec.len() as u32;
         self.u8_vec.extend_from_slice(v);
         let end = self.u8_vec.len() as u32;
-        let r = self.node_vec.len() as u32;
-        self.node_vec.push(NodePtr::Atom(IntAtomBuf { start, end }));
-        r
+        self.atom_vec.push(IntAtomBuf { start, end });
+        -(self.atom_vec.len() as i32)
     }
 
     fn new_pair(&mut self, first: Self::Ptr, rest: Self::Ptr) -> Self::Ptr {
-        let r: u32 = self.node_vec.len() as u32;
-        self.node_vec.push(NodePtr::Pair(first, rest));
+        let r = self.pair_vec.len() as i32;
+        self.pair_vec.push(IntPair { first, rest });
         r
     }
 
     fn atom<'a>(&'a self, node: &'a Self::Ptr) -> &'a [u8] {
-        match self.node_vec[*node as usize] {
-            NodePtr::Atom(IntAtomBuf { start, end }) => &self.u8_vec[start as usize..end as usize],
-            _ => panic!("expected atom, got pair"),
+        if *node >= 0 {
+            panic!("expected atom, got pair");
         }
+        let atom = self.atom_vec[(-*node - 1) as usize];
+        &self.u8_vec[atom.start as usize..atom.end as usize]
     }
 
     fn buf<'a>(&'a self, node: &'a Self::AtomBuf) -> &'a [u8] {
@@ -74,17 +83,20 @@ impl Allocator for IntAllocator {
     }
 
     fn sexp(&self, node: &Self::Ptr) -> SExp<Self::Ptr, Self::AtomBuf> {
-        match self.node_vec[*node as usize] {
-            NodePtr::Atom(atombuf) => SExp::Atom(atombuf),
-            NodePtr::Pair(left, right) => SExp::Pair(left, right),
+        if *node >= 0 {
+            let pair = self.pair_vec[*node as usize];
+            SExp::Pair(pair.first, pair.rest)
+        } else {
+            let atom = self.atom_vec[(-*node - 1) as usize];
+            SExp::Atom(atom)
         }
     }
 
     fn null(&self) -> Self::Ptr {
-        0
+        -1
     }
 
     fn one(&self) -> Self::Ptr {
-        1
+        -2
     }
 }
diff --git a/src/py/run_program.rs b/src/py/run_program.rs
index 0c47bebb..93102ad9 100644
--- a/src/py/run_program.rs
+++ b/src/py/run_program.rs
@@ -4,7 +4,7 @@ use crate::int_allocator::IntAllocator;
 use crate::more_ops::op_unknown;
 use crate::node::Node;
 use crate::py::f_table::{make_f_lookup, FLookup};
-use crate::reduction::{EvalErr, Reduction};
+use crate::reduction::Response;
 use crate::run_program::{run_program, OperatorHandler};
 use crate::serialize::{node_from_bytes, node_to_bytes};
 use lazy_static::lazy_static;
@@ -26,8 +26,8 @@ impl OperatorHandler<IntAllocator> for OperatorHandlerWithMode {
         &self,
         allocator: &mut IntAllocator,
         o: <IntAllocator as Allocator>::AtomBuf,
-        argument_list: &u32,
-    ) -> Result<Reduction<u32>, EvalErr<u32>> {
+        argument_list: &<IntAllocator as Allocator>::Ptr,
+    ) -> Response<<IntAllocator as Allocator>::Ptr> {
         let op = &allocator.buf(&o);
         if op.len() == 1 {
             if let Some(f) = F_TABLE[op[0] as usize] {
@@ -58,11 +58,10 @@ pub fn serialize_and_run_program(
     let f: Box<dyn OperatorHandler<IntAllocator>> = Box::new(OperatorHandlerWithMode {
         strict: (flags & STRICT_MODE) != 0,
     });
-    let program: u32 = node_from_bytes(&mut allocator, program).unwrap();
+    let program = node_from_bytes(&mut allocator, program).unwrap();
+    let args = node_from_bytes(&mut allocator, args).unwrap();
 
-    let args: u32 = node_from_bytes(&mut allocator, args).unwrap();
-
-    let r: Result<Reduction<u32>, EvalErr<u32>> = run_program(
+    let r = run_program(
         &mut allocator,
         &program,
         &args,