diff --git a/generics/generics.js b/generics/generics.js
index 5515b30..a854f80 100644
--- a/generics/generics.js
+++ b/generics/generics.js
@@ -1,5 +1,6 @@
 import { eqType } from "../type.js";
-import { pretty, zip } from "../util.js";
+import { zip } from "../util/util.js";
+import { pretty } from '../util/pretty.js';
 
 // constructor for generic types
 //   for instance, the type:
@@ -22,6 +23,8 @@ export const makeGeneric = callback => {
 
 // From the given set of type variables, return only those that occur in the given type.
 export const occurring = (type, typeVars) => {
+  console.log("occurring", type, typeVars);
+  
   if (typeVars.has(type)) {
     // type IS a type variable:
     return new Set([type]);
diff --git a/generics/generics.test.js b/generics/generics.test.js
index 3054c13..0d967d2 100644
--- a/generics/generics.test.js
+++ b/generics/generics.test.js
@@ -1,7 +1,7 @@
 import { Bool, Int } from "../primitives/types.js";
 import { fnType, lsType } from "../structures/types.js";
 import { assign, makeGeneric, unify } from "./generics.js";
-import { pretty } from "../util.js";
+import { pretty } from "../util/pretty.js";
 
 // a -> Int
 const a_to_Int = makeGeneric(a => fnType(a)(Int));
diff --git a/lib/symbol.js b/lib/symbol.js
new file mode 100644
index 0000000..1b67d34
--- /dev/null
+++ b/lib/symbol.js
@@ -0,0 +1,22 @@
+import { constructSymbol, eqSymbol, getName } from "../primitives/symbol.js";
+import { Bool, SymbolT, Type } from "../primitives/types.js";
+import { String } from "../structures/list.js";
+import { typedFnType } from "../structures/types.js";
+
+export const ModuleSymbol = {l:[
+  {i: SymbolT, t: Type},
+
+  ...typedFnType(constructSymbol, fnType =>
+    fnType
+      (String)
+      (SymbolT)
+  ),
+
+  ...typedFnType(getName, fnType =>
+    fnType
+      (SymbolT)
+      (String)
+  ),
+
+  ...typedFnType(eqSymbol, fnType => fnType(SymbolT, fnType(SymbolT, Bool))),
+]};
diff --git a/lib/type_constructor.js b/lib/type_constructor.js
new file mode 100644
index 0000000..99ee760
--- /dev/null
+++ b/lib/type_constructor.js
@@ -0,0 +1,14 @@
+import { Int, SymbolT, Type } from "../primitives/types.js";
+import { typedFnType } from "../structures/types.js";
+import { makeTypeConstructor } from "../type_constructor.js";
+
+// This function and its type signature cannot be in the same file as 'makeTypeConstructor' because then we get an import cycle among JS modules.
+export const ModuleTypeConstructor = {l:[
+  ...typedFnType(makeTypeConstructor, fnType =>
+    fnType
+      (SymbolT)
+      (fnType
+        (Int)
+        (Type)
+      )),
+]};
diff --git a/primitives/symbol.js b/primitives/symbol.js
new file mode 100644
index 0000000..209c51f
--- /dev/null
+++ b/primitives/symbol.js
@@ -0,0 +1,8 @@
+// The functions are only defined here. For their types, see lib/symbol.js
+
+// The point of having an explicit constructor for SymbolT, is that we can later swap it out for something that is (de-)serializable.
+export const constructSymbol = name => Symbol(name);
+
+export const getName = symbol => symbol.description;
+
+export const eqSymbol = a => b => a === b;
diff --git a/primitives/types.js b/primitives/types.js
index ce2ee79..c798945 100644
--- a/primitives/types.js
+++ b/primitives/types.js
@@ -1,9 +1,17 @@
 // to break up dependency cycles, primitive types are defined in their own JS module
 
-export const Int    = { symbol: Symbol('Int')   , params: [] };
-export const Bool   = { symbol: Symbol('Bool')  , params: [] };
-export const Double = { symbol: Symbol('Double'), params: [] };
-export const Byte   = { symbol: Symbol('Byte')  , params: [] };
-export const Char   = { symbol: Symbol('Char')  , params: [] };
+import { makeTypeConstructor } from "../type_constructor.js";
+import { constructSymbol } from "./symbol.js";
 
-export const Type = { symbol: Symbol('Type'), params: [] };
+export const Int     = makeTypeConstructor(constructSymbol('Int'), 0);
+export const Bool    = makeTypeConstructor(constructSymbol('Bool'), 0);
+export const Double  = makeTypeConstructor(constructSymbol('Double'), 0);
+export const Byte    = makeTypeConstructor(constructSymbol('Byte'), 0);
+export const Char    = makeTypeConstructor(constructSymbol('Char'), 0);
+
+// Unit type has only 1 instance, the empty tuple.
+export const Unit    = makeTypeConstructor(constructSymbol('Unit'), 0);
+
+export const SymbolT = makeTypeConstructor(constructSymbol('Symbol'), 0);
+
+export const Type    = makeTypeConstructor(constructSymbol('Type'), 0);
diff --git a/primitives/unit.js b/primitives/unit.js
new file mode 100644
index 0000000..68fa78c
--- /dev/null
+++ b/primitives/unit.js
@@ -0,0 +1,13 @@
+import { typedFnType } from "../structures/types.js";
+import { Bool, Type, Unit } from "./types.js";
+
+const eqUnit = x => y => x === y;
+
+export const ModuleUnit = {l:[
+  {i: {}, t: Unit},
+
+  {i: Unit, t: Type},
+
+  // Unit -> Unit -> Bool
+  ...typedFnType(eqUnit, fnType => fnType(Unit)(fnType(Unit)(Bool))),
+]};
diff --git a/scripts/int_or_bool.js b/scripts/int_or_bool.js
index 5d6ff67..0a1e1f0 100644
--- a/scripts/int_or_bool.js
+++ b/scripts/int_or_bool.js
@@ -2,7 +2,7 @@ import { assign, makeGeneric, unify } from "../generics/generics.js";
 import { Bool, Int } from "../primitives/types.js";
 import { constructorLeft, constructorRight, match } from "../structures/sum.js";
 import { fnType, sumType } from "../structures/types.js";
-import { pretty } from "../util.js";
+import { pretty } from '../util/pretty.js';
 
 const IntOrBool = sumType(Int)(Bool);
 
diff --git a/scripts/main.js b/scripts/main.js
index 37988f6..af558e9 100644
--- a/scripts/main.js
+++ b/scripts/main.js
@@ -1,10 +1,11 @@
-import { select } from '@inquirer/prompts';
+import { select, number } from '@inquirer/prompts';
 import { ModulePoint } from "../lib/point.js";
-import { DefaultMap, pretty, prettyT } from '../util.js';
-import { symbolFunction } from '../structures/types.js';
+import { DefaultMap } from "../util/defaultmap.js";
+import { pretty } from '../util/pretty.js';
+import { isFunction, prettyT } from '../structures/types.js';
 import { ModuleStd } from '../stdlib.js';
-import { Type } from "../primitives/types.js";
-import { assign, makeGeneric, unify } from '../generics/generics.js';
+import { Double, Int, Type } from "../primitives/types.js";
+import { eqType } from '../type.js';
 
 class Context {
   constructor(mod) {
@@ -21,7 +22,7 @@ class Context {
     this.functionsTo = new DefaultMap(() => new Set()); // type to incoming function
 
     for (const t of this.instances.m.keys()) {
-      if (t.symbol === symbolFunction) {
+      if (isFunction(t)) {
         // 't' is a function signature
         for (const fn of this.instances.getdefault(t)) {
           this.functionsFrom.getdefault(t.params[0], true).add(fn);
@@ -67,7 +68,9 @@ let ctx = new Context({l:[
 const prettyIT = ({i, t}) => ({
   strI: isType(i) ? prettyT(i) : pretty(i),
   strT: prettyT(t),
-})
+  // strI: pretty(i),
+  // strT: pretty(t),
+});
 
 const toChoices = ([i, types]) => {
   return [...types].map(t => {
@@ -131,7 +134,7 @@ async function listAllFunctions() {
       ...[...ctx.types.m.entries()]
         .filter(([i, types]) => {
           for (const type of types) {
-            if (type.symbol === symbolFunction)
+            if (isFunction(type))
               return true;
           }
           return false;
@@ -192,7 +195,7 @@ async function listAllInstances() {
 }
 
 async function instanceOrTypeOrFnOptions({i, t}) {
-  if (t.symbol === symbolFunction) {
+  if (isFunction(t)) {
     return functionOptions(i, t);
   }
   if (isType(i)) {
@@ -257,13 +260,29 @@ async function functionOptions(fn, fnT) {
 async function callFunction(fn, fnT) {
   const {strI, strT} = prettyIT({i: fn, t: fnT});
   const inType = fnT.params[0];
-  const choice = await select({
-    message: `select parameter for function ${strI} :: ${strT}`,
-    choices: [
-      "(go back)",
-      ... [...ctx.instances.getdefault(inType)].flatMap(i => toChoices([i, ctx.types.getdefault(i)])),
-    ],
-  });
+  const choice = await (async () => {
+    if (eqType(inType)(Int)) {
+      const n = await number({
+        message: `enter an integer (leave empty to go back):`,
+        step: 1, // only integers
+      });
+      return (n === undefined) ? "(go back)" : {i: BigInt(n), t: Int};
+    }
+    if (eqType(inType)(Double)) {
+      const n = await number({
+        message: `enter a number (leave empty to go back):`,
+        step: 'any',
+      });
+      return (n === undefined) ? "(go back)" : {i: n, t: Int};
+    }
+    return select({
+      message: `select parameter for function ${strI} :: ${strT}`,
+      choices: [
+        "(go back)",
+        ... [...ctx.instances.getdefault(inType)].flatMap(i => toChoices([i, ctx.types.getdefault(i)])),
+      ],
+    });
+  })();
   if (choice === "(go back)") {
     return;
   }
@@ -296,7 +315,7 @@ async function instanceOptions(i,t) {
 
 async function transform(i, t) {
   const {strI, strT} = prettyIT({i, t});
-  console.log(ctx.functionsFrom.getdefault(t));
+  // console.log(ctx.functionsFrom.getdefault(t));
   
   const choice = await select({
     message: `choose transformation to perform on ${strI} :: ${strT}`,
diff --git a/stdlib.js b/stdlib.js
index e25b914..fda246d 100644
--- a/stdlib.js
+++ b/stdlib.js
@@ -1,8 +1,11 @@
+import { ModuleSymbol } from "./lib/symbol.js";
+import { ModuleTypeConstructor } from "./lib/type_constructor.js";
 import { ModuleBool } from "./primitives/bool.js";
 import { ModuleByte } from "./primitives/byte.js";
 import { ModuleChar } from "./primitives/char.js";
 import { ModuleDouble } from "./primitives/double.js";
 import { ModuleInt } from "./primitives/int.js";
+import { ModuleUnit } from "./primitives/unit.js";
 import { ModuleFunction } from "./structures/function.js";
 import { ModuleList } from "./structures/list.js";
 import { ModuleProduct } from "./structures/product.js";
@@ -14,12 +17,16 @@ export const ModuleStd = {l:[
   ...ModuleType.l,
   ...ModuleTyped.l,
 
+  ...ModuleTypeConstructor.l,
+
   // Primitive types
   ...ModuleBool.l,
   ...ModuleByte.l,
   ...ModuleChar.l,
   ...ModuleDouble.l,
   ...ModuleInt.l,
+  ...ModuleSymbol.l,
+  ...ModuleUnit.l,
 
   // Types that consist of other types
   ...ModuleFunction.l,
diff --git a/structures/list.js b/structures/list.js
index c20e8ce..6582ff9 100644
--- a/structures/list.js
+++ b/structures/list.js
@@ -1,5 +1,5 @@
-import { fnType, typedFnType } from "./types.js";
-import { Type } from "../primitives/types.js";
+import { typedFnType } from "./types.js";
+import { Char, Type } from "../primitives/types.js";
 import { Int } from "../primitives/types.js";
 import { makeGeneric } from "../generics/generics.js";
 import { lsType } from "./types.js";
@@ -10,6 +10,8 @@ const get = ls => i => ls.l[i];
 const put = ls => i => elem => ({l: ls.l.with(Number(i), elem)});
 const push = ls => elem => ({l:ls.l.concat([elem])});
 
+export const String = lsType(Char); // alias
+
 export const ModuleList = {l:[
   // Type -> Type
   ...typedFnType(lsType, fnType =>
@@ -55,4 +57,6 @@ export const ModuleList = {l:[
         )
     )
   ),
+
+  // {i: String, t: Type}, // alias
 ]};
diff --git a/structures/nominal.js b/structures/nominal.js
new file mode 100644
index 0000000..564cea7
--- /dev/null
+++ b/structures/nominal.js
@@ -0,0 +1,13 @@
+import { Any } from "../typed.js";
+import { String } from "./list.js";
+import { sumType, prodType, fnType } from "./types.js";
+
+
+export const createNominalADT = symbol => variants => {
+
+};
+
+export const createNominalADTFnType =
+  fnType
+    (Any)
+    ();
\ No newline at end of file
diff --git a/structures/set.js b/structures/set.js
new file mode 100644
index 0000000..48f52ea
--- /dev/null
+++ b/structures/set.js
@@ -0,0 +1,38 @@
+import { setType, typedFnType } from "./types.js";
+import { Bool, Type } from "../primitives/types.js";
+import { makeGeneric } from "../generics/generics.js";
+
+// 'normal' implementation
+const emptySet = new Set();
+const has = set => elem => set.has(elem);
+const add = set => elem => new Set([...set, elem]);
+
+export const ModuleList = {l:[
+  // Type -> Type
+  ...typedFnType(setType, fnType =>
+    fnType
+      /* in  */ (Type)
+      /* out */ (Type)
+  ),
+
+  {i: emptySet, t: makeGeneric(a => setType(a))},
+
+  ...typedFnType(has, fnType =>
+    makeGeneric(a =>
+      fnType
+        /* in  */ (setType(a))
+        /* out */ (fnType
+          /* in  */ (a)
+          /* out */ (Bool)
+        ))),
+
+  ...typedFnType(add, fnType =>
+    makeGeneric(a =>
+      fnType
+        /* in  */ (setType(a))
+        /* out */ (fnType
+          /* in  */ (a)
+          /* out */ (setType(a))
+        ))),
+
+]};
diff --git a/structures/types.js b/structures/types.js
index 3e7056c..a435d27 100644
--- a/structures/types.js
+++ b/structures/types.js
@@ -1,21 +1,17 @@
 // to break up dependency cycles, type constructors are defined in their own JS module
 
 import { Type } from "../primitives/types.js";
-import { DefaultMap } from "../util.js";
-
+import { getSymbol, makeTypeConstructor } from "../type_constructor.js";
 
 // Function type
 
 // The registry ensures that we never accidentally create more than one JS object for the same function type.
 // It is a cheap workaround for JS lacking customizable hash-functions and equality-testing-functions.
 // This same pattern is repeated throughout the code for all non-nullary type constructors (list, sum, product, ...)
-export const symbolFunction = Symbol('Function');
-const fnTypeRegistry = new DefaultMap(inType => new DefaultMap(outType => ({
-  symbol: symbolFunction,
-  params: [inType, outType],
-})));
-// type constructor
-export const fnType = inType => outType => fnTypeRegistry.getdefault(inType, true).getdefault(outType, true);
+const symbolFunction = Symbol('Function');
+export const fnType = makeTypeConstructor(symbolFunction, 2);
+
+export const isFunction = type => getSymbol(type) === symbolFunction;
 
 // Convenience function. Wrapper around function below.
 export const typedFnType = (instance, callback) => {
@@ -41,35 +37,48 @@ export const typedFnType2 = callback => {
   ];
 };
 
-
 // Sum type
 
-export const symbolSum = Symbol("Sum");
-const sumTypeRegistry = new DefaultMap(leftType => new DefaultMap(rightType => ({
-  symbol: symbolSum,
-  params: [leftType, rightType],
-})));
-// type constructor
-export const sumType = leftType => rightType => sumTypeRegistry.getdefault(leftType, true).getdefault(rightType, true);
-
+const symbolSum = Symbol("Sum");
+export const sumType = makeTypeConstructor(symbolSum, 2);
 
 // Product type
 
-export const symbolProduct = Symbol("Product");
-const productTypeRegistry = new DefaultMap(leftType => new DefaultMap(rightType => ({
-  symbol: symbolProduct,
-  params: [leftType, rightType],
-})));
-// type constructor
-export const prodType = leftType => rightType => productTypeRegistry.getdefault(leftType, true).getdefault(rightType, true);
-
+const symbolProduct = Symbol("Product");
+export const prodType = makeTypeConstructor(symbolProduct, 2);
 
 // List type
 
-export const symbolList = Symbol('List');
-const listTypeRegistry = new DefaultMap(elementType => ({
-  symbol: symbolList,
-  params: [elementType],
-}));
-// type constructor
-export const lsType = elementType => listTypeRegistry.getdefault(elementType, true);
+const symbolList = Symbol('List');
+export const lsType = makeTypeConstructor(symbolList, 1);
+
+// Set type
+
+const symbolSet = Symbol('Set');
+export const setType = makeTypeConstructor(symbolSet, 1);
+
+// Pretty print type
+export function prettyT(type) {
+  if (type.typeVars) {
+    if (type.typeVars.size > 0) {
+      return `∀${[...type.typeVars].map(prettyT).join(", ")}: ${prettyT(type.type)}`;
+    }
+    return prettyT(type.type);
+  }
+  if (type.symbol === symbolFunction) {
+    return `${prettyT(type.params[0])} -> ${prettyT(type.params[1])}`;
+  }
+  if (type.symbol === symbolList) {
+    return `[${prettyT(type.params[0])}]`;
+  }
+  if (type.symbol === symbolProduct) {
+    return `(${prettyT(type.params[0])}, ${prettyT(type.params[1])})`;
+  }
+  if (type.symbol === symbolSum) {
+    return `(${prettyT(type.params[0])} | ${prettyT(type.params[1])})`;
+  }
+  if (type.params.length === 0) {
+    return type.symbol.description;
+  }
+  return `${type.symbol.description}(${type.params.map(prettyT).join(", ")})`;
+}
diff --git a/structures/versioned.js b/structures/versioned.js
new file mode 100644
index 0000000..f7f20a1
--- /dev/null
+++ b/structures/versioned.js
@@ -0,0 +1,63 @@
+import { makeGeneric } from "../generics/generics.js";
+import { Bool } from "../primitives/types.js";
+import { makeTypeConstructor } from "../type_constructor.js";
+import { getEq } from "../typeclasses/eq.js";
+import { eqDictType } from "../typeclasses/eq_type.js";
+import { fnType, setType } from "./types.js";
+
+const symbolVersioned = Symbol("Versioned");
+export const versionedType = makeTypeConstructor(symbolVersioned, 1);
+
+
+export const constructor = parents => value  => {
+  return { parents, alternatives };
+}
+
+const constructorType = makeGeneric(a =>
+  fnType
+    (a)
+    (setType(versionedType(a)))
+);
+
+// const getValue = v => 
+
+const eq = eqDict => vA => vB => {
+  return getEq(eqDict)(vA.value,vB.value) // compare values
+      && (vA.parents.symmetricDifference(vB.parents).size === 0); // compare parents
+}
+
+// EqDict a -> Versioned a -> Versioned a -> Bool
+const eqVersioned = makeGeneric(a =>
+  fnType
+    (eqDictType(a))
+    (fnType
+      (versionedType(a))
+      (fnType
+        (versionedType(a))
+        (Bool)
+      )
+    )
+);
+
+// EqDict a -> Versioned a -> Versioned a -> Versioned a -> Versioned a
+export const mergeThreeWay = eqDict => vLCA => vA => vB => {
+  if (eq(eqDict)(vLCA)(vA)) {
+    return vB; // vB successor of vA
+  }
+  if (eq(eqDict)(vLCA)(vB)) {
+    return vA; // vA successor of vB
+  }
+  return mergeConcurrent(vLCA)(vA)(vB);
+};
+
+export const mergePrimitives = vA => vB => vA.union(vB);
+
+// Versioned a -> Versioned a -> Versioned a
+export const mergePrimitivesType = makeGeneric(a => 
+  fnType
+    (versionedType(a))
+    (fnType
+      (versionedType(a))
+      (versionedType(a))
+    )
+);
\ No newline at end of file
diff --git a/type.js b/type.js
index 4d13737..bbd299f 100644
--- a/type.js
+++ b/type.js
@@ -1,12 +1,10 @@
-import { Bool, Type } from "./primitives/types.js";
-import { typedFnType } from "./structures/types.js";
-import { deepEqual } from "./util.js";
-
-export const getSymbol = type => type.symbol;
-export const getParams = type => type.params;
+import { Bool, SymbolT, Type } from "./primitives/types.js";
+import { isFunction, lsType, typedFnType } from "./structures/types.js";
+import { getSymbol, getParams } from "./type_constructor.js";
+import { deepEqual } from "./util/util.js";
 
 // we can test whether types are equal:
-export const eqType = deepEqual;
+export const eqType = t1 => t2 => deepEqual(t1, t2);
 
 // a module is just a set of typed objects
 // each 'typed object' is implicitly an instance of TypeLink (defined below)
@@ -17,13 +15,8 @@ export const ModuleType = {l:[
   //    ...
   // see: https://lean-lang.org/functional_programming_in_lean/functor-applicative-monad/universes.html
 
-  // Type :: Type
   {i: Type, t: Type},
 
-  // ...typedFnType(getSymbol, fnType => fnType({in: Type, out: Int})),
-
-  // ...typedFnType(getParams, fnType => fnType({in: Type, out: lsType(Type)})),
-
   // Type -> Type -> Bool
   ...typedFnType(eqType, fnType =>
     fnType
@@ -32,4 +25,9 @@ export const ModuleType = {l:[
         (Type)
         (Bool)
       )),
+  
+  ...typedFnType(getSymbol, fnType => fnType(Type)(SymbolT)),
+  ...typedFnType(getParams, fnType => fnType(Type)(lsType(Type))),
+
+  ...typedFnType(isFunction, fnType => fnType(Type)(Bool)),
 ]};
diff --git a/type_constructor.js b/type_constructor.js
new file mode 100644
index 0000000..8fe9925
--- /dev/null
+++ b/type_constructor.js
@@ -0,0 +1,15 @@
+import { DefaultMap } from "./util/defaultmap.js";
+
+// Creates a new nominal type
+export const makeTypeConstructor = (name, n_ary, params = []) => {
+  if (n_ary === 0) {
+    return { symbol: Symbol(name), params };
+  }
+  else {
+    const m = new DefaultMap(typeParam => makeTypeConstructor(name, n_ary - 1, params.concat([typeParam])));
+    return typeParam => m.getdefault(typeParam, true);
+  }
+};
+
+export const getSymbol = type => type.symbol;
+export const getParams = type => ({ l: type.params });
diff --git a/typeclasses/eq.js b/typeclasses/eq.js
index 14752a1..64e2b6d 100644
--- a/typeclasses/eq.js
+++ b/typeclasses/eq.js
@@ -1,8 +1,8 @@
 import { makeGeneric } from "../generics/generics";
-import { Type } from "../primitives/types";
+import { SymbolT, Type, Unit } from "../primitives/types";
 import { typedFnType } from "../structures/types";
 import { Bool, Byte, Char, Double, Int } from "../primitives/types";
-import { deepEqual } from "../util";
+import { deepEqual } from "../util/util";
 import { eqDictType } from "./eq_type";
 
 export const getEq = numDict => numDict.eq;
@@ -32,10 +32,12 @@ const eq = x => y => deepEqual(x,y);
 const EqDict = {eq};
 
 export const EqInstances = new Map([
-  [Int   , EqDict],
-  [Bool  , EqDict],
-  [Double, EqDict],
-  [Byte  , EqDict],
-  [Char  , EqDict],
-  [Type  , EqDict],
+  [Int    , EqDict],
+  [Bool   , EqDict],
+  [Double , EqDict],
+  [Byte   , EqDict],
+  [Char   , EqDict],
+  [Unit   , EqDict],
+  [Type   , EqDict],
+  [SymbolT, EqDict],
 ]);
diff --git a/typeclasses/eq_type.js b/typeclasses/eq_type.js
index 5055d66..309a46d 100644
--- a/typeclasses/eq_type.js
+++ b/typeclasses/eq_type.js
@@ -1,4 +1,4 @@
-import { DefaultMap } from "../util.js";
+import { DefaultMap } from "../util/defaultmap.js";
 
 const eqDictSymbol = Symbol('EqDict');
 const eqDictTypeRegistry = new DefaultMap(a => ({
diff --git a/typeclasses/num.test.js b/typeclasses/num.test.js
index c0ffcbc..e2200ba 100644
--- a/typeclasses/num.test.js
+++ b/typeclasses/num.test.js
@@ -2,7 +2,7 @@ import { assign } from "../generics/generics.js";
 import { makeGeneric } from "../generics/generics.js";
 import { Double, Int } from "../primitives/types.js";
 import { fnType } from "../structures/types.js";
-import { pretty } from "../util.js";
+import { pretty } from '../util/pretty.js';
 import { getMul, NumInstances } from "./num.js";
 import { numDictType } from "./num_type.js";
 
diff --git a/typeclasses/num_type.js b/typeclasses/num_type.js
index 989040e..db6693d 100644
--- a/typeclasses/num_type.js
+++ b/typeclasses/num_type.js
@@ -1,4 +1,4 @@
-import { DefaultMap } from "../util.js";
+import { DefaultMap } from "../util/defaultmap.js";
 
 const numDictSymbol = Symbol("NumDict");
 const numDictTypeRegistry = new DefaultMap(a => ({
diff --git a/util.js b/util.js
deleted file mode 100644
index c8c7214..0000000
--- a/util.js
+++ /dev/null
@@ -1,79 +0,0 @@
-import { inspect } from 'node:util';
-import { symbolFunction, symbolList, symbolProduct, symbolSum } from './structures/types.js';
-
-// re-inventing the wheel:
-export function deepEqual(a, b) {
-  if (a === b) return true; // <- shallow equality and primitives
-  if (typeof a !== 'object' || typeof b !== 'object' || a === null || b === null) {
-    return false;
-  }
-  if (Array.isArray(a) && Array.isArray(b)) {
-    if (a.length !== b.length) return false;
-    for (let i = 0; i < a.length; i++) {
-      if (!deepEqual(a[i], b[i])) return false;
-    }
-    return true;
-  }
-  const keysA = Object.keys(a);
-  const keysB = Object.keys(b);
-  if (keysA.length !== keysB.length) return false;
-  for (let key of keysA) {
-    if (!keysB.includes(key) || !deepEqual(a[key], b[key])) {
-      return false;
-    }
-  }
-  return true;
-}
-
-// zip two arrays
-export function zip(a, b) {
-  return a.map((k, i) => [k, b[i]]);
-}
-
-export class DefaultMap {
-  constructor(defaultValue, ...rest) {
-    this.defaultValue = defaultValue;
-    this.m = new Map(rest);
-  }
-  getdefault(key, addToMapping=false) {
-    return this.m.get(key) || (() => {
-      const val = this.defaultValue(key);
-      if (addToMapping)
-        this.m.set(key, val);
-      return val;
-    })();
-  }
-  entries() {
-    return this.m.entries();
-  }
-}
-
-export function pretty(obj) {
-  return inspect(obj, {colors: true, depth: null});
-}
-
-// Pretty print type
-export function prettyT(type) {
-  if (type.typeVars) {
-    if (type.typeVars.size > 0) {
-      return `∀${[...type.typeVars].map(prettyT).join(", ")}: ${prettyT(type.type)}`;
-    }
-    return prettyT(type.type);
-  }
-  if (type.symbol === symbolFunction) {
-    return `${prettyT(type.params[0])} -> ${prettyT(type.params[1])}`;
-  }
-  if (type.symbol === symbolList) {
-    return `[${prettyT(type.params[0])}]`;
-  }
-  if (type.symbol === symbolProduct) {
-    return `(${prettyT(type.params[0])}, ${prettyT(type.params[1])})`;
-  }
-  if (type.symbol === symbolSum) {
-    return `(${prettyT(type.params[0])} | ${prettyT(type.params[1])})`;
-  }
-  if (type.params.length === 0) {
-    return type.symbol.description;
-  }
-  return `${type.symbol.description}(${type.params.map(prettyT).join(", ")})`;
-}
diff --git a/util/defaultmap.js b/util/defaultmap.js
new file mode 100644
index 0000000..6d40c66
--- /dev/null
+++ b/util/defaultmap.js
@@ -0,0 +1,18 @@
+
+export class DefaultMap {
+  constructor(defaultValue, ...rest) {
+    this.defaultValue = defaultValue;
+    this.m = new Map(rest);
+  }
+  getdefault(key, addToMapping = false) {
+    return this.m.get(key) || (() => {
+      const val = this.defaultValue(key);
+      if (addToMapping)
+        this.m.set(key, val);
+      return val;
+    })();
+  }
+  entries() {
+    return this.m.entries();
+  }
+}
diff --git a/util/pretty.js b/util/pretty.js
new file mode 100644
index 0000000..d343a79
--- /dev/null
+++ b/util/pretty.js
@@ -0,0 +1,7 @@
+import { inspect } from 'node:util';
+
+
+export function pretty(obj) {
+  return inspect(obj, { colors: true, depth: null });
+}
+
diff --git a/util/util.js b/util/util.js
new file mode 100644
index 0000000..3d6a002
--- /dev/null
+++ b/util/util.js
@@ -0,0 +1,31 @@
+
+// re-inventing the wheel:
+export function deepEqual(a, b) {
+  if (a === b) return true; // <- shallow equality and primitives
+  if (typeof a !== 'object' || typeof b !== 'object' || a === null || b === null) {
+    return false;
+  }
+  if (Array.isArray(a) && Array.isArray(b)) {
+    if (a.length !== b.length) return false;
+    for (let i = 0; i < a.length; i++) {
+      if (!deepEqual(a[i], b[i])) return false;
+    }
+    return true;
+  }
+  const keysA = Object.keys(a);
+  const keysB = Object.keys(b);
+  if (keysA.length !== keysB.length) return false;
+  for (let key of keysA) {
+    if (!keysB.includes(key) || !deepEqual(a[key], b[key])) {
+      return false;
+    }
+  }
+  return true;
+}
+
+// zip two arrays
+export function zip(a, b) {
+  return a.map((k, i) => [k, b[i]]);
+}
+
+