diff --git a/lib/util/fixpoint.js b/lib/util/fixpoint.js
index c149fce..7014035 100644
--- a/lib/util/fixpoint.js
+++ b/lib/util/fixpoint.js
@@ -4,10 +4,12 @@
 //   FRANCOIS POTTIER,<a href="https://gallium.inria.fr/~fpottier/publis/fpottier-fix.pdf"> Lazy Least Fixed Points in ML</a>
 //   <a href="https://gitlab.inria.fr/fpottier/fix">OCAML Code</a>
 
-export const fixer = (eqsFunction, lattice) => {
-  const mFixed = new Map();
-  const mTransient = new Map();
-  const mParents = new Map();
+// eqsFunction: V -> (V -> P) -> P
+// elemCmp: (V -> V -> Ordering)
+export const fixer = (eqsFunction, dictType, lattice) => {
+  const mFixed = dictType();
+  const mTransient = dictType();
+  const mParents = dictType();
 
   const mWorkset = []; // Note: we use a Stack (LIFO), but FIFO queue or any other collection would also work.
 
@@ -16,7 +18,6 @@ export const fixer = (eqsFunction, lattice) => {
   const ensureTransient = node => {
     if (mTransient.has(node)) return;
     mTransient.set(node, lattice.bottom());
-    mParents.set(node, []);
     mWorkset.push(node);
   }
 
@@ -47,7 +48,7 @@ export const fixer = (eqsFunction, lattice) => {
       }
 
       if (!lattice.equality(mTransient.get(current), newProperty)) {
-        mTransient.put(current, newProperty);
+        mTransient.set(current, newProperty);
         mWorkset.push(...(mParents.get(current) || []));
       }
     }
diff --git a/lib/util/rbtree_wrapper.js b/lib/util/rbtree_wrapper.js
index dbc3fce..b9c1682 100644
--- a/lib/util/rbtree_wrapper.js
+++ b/lib/util/rbtree_wrapper.js
@@ -29,3 +29,46 @@ export class RBTreeWrapper {
     return this.tree.keys.map(key => [key, this.tree.get(key)]);
   }
 }
+
+// Can be used as drop-in replacement for Map
+// Why create a mutable adapter for a purely-functional data structure?
+// Because the builtin Map does not allow custom comparison functions and this one does.
+export class MutableRBTree {
+  constructor(tree, printf = defaultPrintf) {
+    this.tree = tree;
+    this[inspect.custom] = printf;
+  }
+
+  static new(cmp) {
+    return new MutableRBTree(createRBTree(cmp));
+  }
+
+  set(key, value) {
+    this.tree = this.tree.remove(key).insert(key, value);
+  }
+
+  delete(key) {
+    this.tree = this.tree.remove(key);
+  }
+
+  get(key) {
+    return this.tree.get(key);
+  }
+
+  has(key) {
+    return this.tree.get(key) !== undefined;
+  }
+
+  clear() {
+    this.tree = createRBTree(this.tree._compare);
+  }
+
+  *[Symbol.iterator]() {
+    const iter = this.tree.begin;
+    while (iter !== undefined && iter.valid) {
+      yield [iter.key, iter.value];
+      iter.next();
+    }
+  }
+}
+
diff --git a/tests/fixpoint.js b/tests/fixpoint.js
index 8942723..70bd6fc 100644
--- a/tests/fixpoint.js
+++ b/tests/fixpoint.js
@@ -1,4 +1,6 @@
+import { compareStrings } from "../lib/compare/primitives.js";
 import { fixer } from "../lib/util/fixpoint.js";
+import { MutableRBTree } from "../lib/util/rbtree_wrapper.js";
 
 const booleanLattice = {
   isMaximal: (value) => value,
@@ -7,7 +9,7 @@ const booleanLattice = {
 };
 
 const nullable = grammar => nonterminal => request => {
-  console.log(nonterminal);
+    console.log(nonterminal);
   const f = nt => {
     return {
       Epsilon: () => true,
@@ -15,7 +17,7 @@ const nullable = grammar => nonterminal => request => {
       NT: () => request(nt.nt),
       Seq: () => f(nt.l) && f(nt.r),
       Alt: () => f(nt.l) || f(nt.r),
-    }[nt.kind]();  
+    }[nt.kind]();
   }
   return f(grammar[nonterminal]);
 }
@@ -65,8 +67,10 @@ const exampleGrammar = {
 // B is nullable because B → A C, and both A and C are nullable.
 // S is nullable because S → A B C, and A, B, and C are all nullable.
 
-
-const isNullable = fixer(nullable(exampleGrammar), booleanLattice);
+const isNullable = fixer(
+  nullable(exampleGrammar), // eqs
+  () => MutableRBTree.new((a,b) => compareStrings(a)(b)), // dict to use
+  booleanLattice);
 
 console.log(isNullable("S"));
 console.log(isNullable("A"));