diff --git a/generic.js b/generic.js
index 259065a..46c136c 100644
--- a/generic.js
+++ b/generic.js
@@ -2,16 +2,35 @@ import { Bool, Int } from "./primitives/symbols.js";
 import { fnType, lsType } from "./type_registry.js";
 import { deepEqual } from "./util.js";
 
-// let nextSymbol = 'a';
+export const makeGeneric = callback => {
+  // type variables to make available:
+  const a = Symbol('a');
+  const b = Symbol('b');
+  const c = Symbol('c');
+  const d = Symbol('d');
+  const e = Symbol('e');
+  const type = callback(a,b,c,d,e);
+  return {
+    typeVars: occurring(type, new Set([a,b,c,d,e])),
+    type,
+  };
+};
 
-// export const makeGeneric = callback => {
-//   const typeParam = Symbol(nextSymbol);
-//   nextSymbol = String.fromCharCode(nextSymbol.charCodeAt(0) + 1);
-//   return {
-//     typeParam,
-//     type: callback(typeParam),
-//   };
-// };
+const occurring = (type, typeVars) => {
+  if (typeVars.has(type)) {
+    return new Set([type]);
+  }
+  if (type.in !== undefined) {
+    // function type
+    return new Set([
+      ...occurring(type.in, typeVars),
+      ...occurring(type.out, typeVars)]);
+  }
+  if (type.listOf !== undefined) {
+    return occurring(type.listOf, typeVars);
+  }
+  return new Set();
+}
 
 
 import { inspect } from 'node:util';
@@ -119,7 +138,7 @@ export const substitute = (type, substitutions) => {
       out: substitute(type.out, substitutions),
     })
   }
-  throw new Error("i don't know what to do :(");
+  return type;
 }
 
 export const assign = (genFnType, paramType) => {
@@ -134,39 +153,26 @@ export const assign = (genFnType, paramType) => {
   };
 }
 
-const a = Symbol('a');
-const formal = {
-  typeVars: new Set([a]),
-  type: fnType({in: a, out: Int}),
-};
-const actual = fnType({in: lsType(Bool), out: Int});
-console.log(matchConcrete(formal, actual));
+// a -> Int
+const a_to_Int = makeGeneric(a => fnType({in: a, out: Int}));
+const Bool_to_Int = fnType({in: lsType(Bool), out: Int});
+console.log(matchConcrete(a_to_Int, Bool_to_Int));
 
+// (a -> a) -> b
+const fnType2 = makeGeneric((a,b) => fnType({in: fnType({in: a, out: a}), out: b}));
+// (Bool -> Bool) -> a
+const fnType3 = makeGeneric(a => fnType({in: fnType({in: Bool, out: Bool}), out: a}));
+console.log(matchGeneric(fnType2, fnType3));
 
-const b = Symbol('b');
-const c = Symbol('c');
-const formal2 = {
-  typeVars: new Set([a, b]),
-  type: fnType({in: fnType({in: a, out: a}), out: b}),
-}
-const actual2 = {
-  typeVars: new Set([c]),
-  type: fnType({in: fnType({in: Bool, out: Bool}), out: c}),
-}
-console.log(matchGeneric(formal2, actual2));
-
-
-const mapFnType = {
-  typeVars: new Set([a, b]),
-  type: fnType({
+// (a -> b) -> [a] -> [b]
+const mapFnType = makeGeneric((a,b) =>
+  fnType({
     in: fnType({in: a, out: b}),
     out: fnType({in: lsType(a), out: lsType(b)}),
-  }),
-};
-const idFnType = {
-  typeVars: new Set([c]),
-  type: fnType({in: c, out: c}),
-};
+  }));
+// a -> a
+const idFnType = makeGeneric(a =>
+  fnType({in: a, out: a}));
 
-// should be: listOf(c) -> listOf(c)
+// should be: [a] -> [a]
 console.log(assign(mapFnType, idFnType));