diff --git a/packages/gp/src/api.ts b/packages/gp/src/api.ts
index f62f56683f..5331a64e95 100644
--- a/packages/gp/src/api.ts
+++ b/packages/gp/src/api.ts
@@ -1,10 +1,7 @@
 import type { Fn, Fn2 } from "@thi.ng/api";
 import type { IRandom } from "@thi.ng/random";
 
-export enum GeneType {
-    TERMINAL,
-    OP,
-}
+export type GeneType = "term" | "op";
 
 /**
  * Type alias for S-expression based AST
@@ -66,12 +63,12 @@ export interface MEPOpts<OP, T> extends GPOpts<OP, T, number> {
 }
 
 export interface TerminalGene<T> {
-    type: GeneType.TERMINAL;
+    type: "term";
     value: T;
 }
 
 export interface OpGene<OP, A> {
-    type: GeneType.OP;
+    type: "op";
     op: OP;
     args: A[];
 }
diff --git a/packages/gp/src/ast.ts b/packages/gp/src/ast.ts
index b79cbbcc54..cf43553116 100644
--- a/packages/gp/src/ast.ts
+++ b/packages/gp/src/ast.ts
@@ -2,7 +2,7 @@ import { assert } from "@thi.ng/api";
 import { SYSTEM } from "@thi.ng/random";
 import { iterate, iterator, repeatedly, takeWhile } from "@thi.ng/transducers";
 import { Location, zipper } from "@thi.ng/zipper";
-import { ASTNode, ASTOpts, GeneType, OpGene } from "./api";
+import type { ASTNode, ASTOpts, OpGene } from "./api";
 import { opNode, probabilities, terminalNode } from "./utils";
 
 export class AST<OP, T> {
@@ -136,7 +136,7 @@ export class AST<OP, T> {
     protected asZipper(tree: ASTNode<OP, T>) {
         return zipper<ASTNode<OP, T>>(
             {
-                branch: (x) => x.type === GeneType.OP,
+                branch: (x) => x.type === "op",
                 children: (x) => (<OpGene<OP, ASTNode<OP, T>>>x).args,
                 factory: (n, args) =>
                     opNode((<OpGene<OP, ASTNode<OP, T>>>n).op, args),
diff --git a/packages/gp/src/mep.ts b/packages/gp/src/mep.ts
index e33246b8c1..0dcf07a720 100644
--- a/packages/gp/src/mep.ts
+++ b/packages/gp/src/mep.ts
@@ -1,7 +1,7 @@
 import { inRange } from "@thi.ng/math";
 import { SYSTEM } from "@thi.ng/random";
 import { repeatedly } from "@thi.ng/transducers";
-import { ASTNode, GeneType, MEPChromosome, MEPGene, MEPOpts } from "./api";
+import type { ASTNode, MEPChromosome, MEPGene, MEPOpts } from "./api";
 import { opNode, probabilities, terminalNode } from "./utils";
 
 export class MEP<OP, T> {
@@ -55,7 +55,7 @@ export class MEP<OP, T> {
         const depths: number[] = [];
         for (let i = 0; i < chromosome.length; i++) {
             const gene = chromosome[i];
-            if (gene.type == GeneType.TERMINAL) {
+            if (gene.type == "term") {
                 res[i] = gene;
                 depths[i] = 1;
             } else {
diff --git a/packages/gp/src/utils.ts b/packages/gp/src/utils.ts
index 7357937984..fba40a2aba 100644
--- a/packages/gp/src/utils.ts
+++ b/packages/gp/src/utils.ts
@@ -1,14 +1,14 @@
 import { assert } from "@thi.ng/api";
 import { add, choices, range } from "@thi.ng/transducers";
-import { GeneType, GPOpts, OpGene, TerminalGene } from "./api";
+import type { GPOpts, OpGene, TerminalGene } from "./api";
 
 export const terminalNode = <T>(value: T): TerminalGene<T> => ({
-    type: GeneType.TERMINAL,
+    type: "term",
     value,
 });
 
 export const opNode = <OP, A>(op: OP, args: A[]): OpGene<OP, A> => ({
-    type: GeneType.OP,
+    type: "op",
     op,
     args,
 });
diff --git a/packages/gp/test/ast.ts b/packages/gp/test/ast.ts
index 001dd5b2af..021e29df8d 100644
--- a/packages/gp/test/ast.ts
+++ b/packages/gp/test/ast.ts
@@ -23,33 +23,33 @@ describe("gp (ast)", () => {
 
     it("generate", () => {
         assert.deepStrictEqual(ast.randomAST(), {
-            type: 1,
+            type: "op",
             op: "+",
             args: [
                 {
-                    type: 1,
+                    type: "op",
                     op: "-",
                     args: [
                         {
-                            type: 0,
+                            type: "term",
                             value: 5,
                         },
                         {
-                            type: 0,
+                            type: "term",
                             value: 1,
                         },
                     ],
                 },
                 {
-                    type: 1,
+                    type: "op",
                     op: "*",
                     args: [
                         {
-                            type: 0,
+                            type: "term",
                             value: 8,
                         },
                         {
-                            type: 0,
+                            type: "term",
                             value: 3,
                         },
                     ],
diff --git a/packages/gp/test/mep.ts b/packages/gp/test/mep.ts
index 1f963abb91..ffc2030372 100644
--- a/packages/gp/test/mep.ts
+++ b/packages/gp/test/mep.ts
@@ -23,121 +23,121 @@ describe("gp (mep)", () => {
 
     it("generate", () => {
         assert.deepStrictEqual(ast.randomChromosome(), [
-            { type: 0, value: 5 },
-            { type: 0, value: 5 },
-            { type: 1, op: "*", args: [1, 1] },
-            { type: 1, op: "-", args: [1, 2] },
-            { type: 1, op: "-", args: [0, 0] },
-            { type: 1, op: "-", args: [3, 4] },
-            { type: 1, op: "*", args: [4, 0] },
-            { type: 1, op: "-", args: [2, 3] },
-            { type: 1, op: "/", args: [1, 4] },
-            { type: 1, op: "-", args: [5, 0] },
+            { type: "term", value: 5 },
+            { type: "term", value: 5 },
+            { type: "op", op: "*", args: [1, 1] },
+            { type: "op", op: "-", args: [1, 2] },
+            { type: "op", op: "-", args: [0, 0] },
+            { type: "op", op: "-", args: [3, 4] },
+            { type: "op", op: "*", args: [4, 0] },
+            { type: "op", op: "-", args: [2, 3] },
+            { type: "op", op: "/", args: [1, 4] },
+            { type: "op", op: "-", args: [5, 0] },
         ]);
     });
 
     it("decode", () => {
         assert.deepStrictEqual(ast.decodeChromosome(ast.randomChromosome()), [
-            { type: 0, value: 5 },
-            { type: 0, value: 5 },
+            { type: "term", value: 5 },
+            { type: "term", value: 5 },
             {
-                type: 1,
+                type: "op",
                 op: "*",
                 args: [
-                    { type: 0, value: 5 },
-                    { type: 0, value: 5 },
+                    { type: "term", value: 5 },
+                    { type: "term", value: 5 },
                 ],
             },
             {
-                type: 1,
+                type: "op",
                 op: "-",
                 args: [
-                    { type: 0, value: 5 },
+                    { type: "term", value: 5 },
                     {
-                        type: 1,
+                        type: "op",
                         op: "*",
                         args: [
-                            { type: 0, value: 5 },
-                            { type: 0, value: 5 },
+                            { type: "term", value: 5 },
+                            { type: "term", value: 5 },
                         ],
                     },
                 ],
             },
             {
-                type: 1,
+                type: "op",
                 op: "-",
                 args: [
-                    { type: 0, value: 5 },
-                    { type: 0, value: 5 },
+                    { type: "term", value: 5 },
+                    { type: "term", value: 5 },
                 ],
             },
             {
-                type: 1,
+                type: "op",
                 op: "-",
                 args: [
                     {
-                        type: 1,
+                        type: "op",
                         op: "-",
                         args: [
-                            { type: 0, value: 5 },
+                            { type: "term", value: 5 },
                             {
-                                type: 1,
+                                type: "op",
                                 op: "*",
                                 args: [
-                                    { type: 0, value: 5 },
-                                    { type: 0, value: 5 },
+                                    { type: "term", value: 5 },
+                                    { type: "term", value: 5 },
                                 ],
                             },
                         ],
                     },
                     {
-                        type: 1,
+                        type: "op",
                         op: "-",
                         args: [
-                            { type: 0, value: 5 },
-                            { type: 0, value: 5 },
+                            { type: "term", value: 5 },
+                            { type: "term", value: 5 },
                         ],
                     },
                 ],
             },
             {
-                type: 1,
+                type: "op",
                 op: "*",
                 args: [
                     {
-                        type: 1,
+                        type: "op",
                         op: "-",
                         args: [
-                            { type: 0, value: 5 },
-                            { type: 0, value: 5 },
+                            { type: "term", value: 5 },
+                            { type: "term", value: 5 },
                         ],
                     },
-                    { type: 0, value: 5 },
+                    { type: "term", value: 5 },
                 ],
             },
             {
-                type: 1,
+                type: "op",
                 op: "-",
                 args: [
                     {
-                        type: 1,
+                        type: "op",
                         op: "*",
                         args: [
-                            { type: 0, value: 5 },
-                            { type: 0, value: 5 },
+                            { type: "term", value: 5 },
+                            { type: "term", value: 5 },
                         ],
                     },
                     {
-                        type: 1,
+                        type: "op",
                         op: "-",
                         args: [
-                            { type: 0, value: 5 },
+                            { type: "term", value: 5 },
                             {
-                                type: 1,
+                                type: "op",
                                 op: "*",
                                 args: [
-                                    { type: 0, value: 5 },
-                                    { type: 0, value: 5 },
+                                    { type: "term", value: 5 },
+                                    { type: "term", value: 5 },
                                 ],
                             },
                         ],
@@ -145,54 +145,54 @@ describe("gp (mep)", () => {
                 ],
             },
             {
-                type: 1,
+                type: "op",
                 op: "/",
                 args: [
-                    { type: 0, value: 5 },
+                    { type: "term", value: 5 },
                     {
-                        type: 1,
+                        type: "op",
                         op: "-",
                         args: [
-                            { type: 0, value: 5 },
-                            { type: 0, value: 5 },
+                            { type: "term", value: 5 },
+                            { type: "term", value: 5 },
                         ],
                     },
                 ],
             },
             {
-                type: 1,
+                type: "op",
                 op: "-",
                 args: [
                     {
-                        type: 1,
+                        type: "op",
                         op: "-",
                         args: [
                             {
-                                type: 1,
+                                type: "op",
                                 op: "-",
                                 args: [
-                                    { type: 0, value: 5 },
+                                    { type: "term", value: 5 },
                                     {
-                                        type: 1,
+                                        type: "op",
                                         op: "*",
                                         args: [
-                                            { type: 0, value: 5 },
-                                            { type: 0, value: 5 },
+                                            { type: "term", value: 5 },
+                                            { type: "term", value: 5 },
                                         ],
                                     },
                                 ],
                             },
                             {
-                                type: 1,
+                                type: "op",
                                 op: "-",
                                 args: [
-                                    { type: 0, value: 5 },
-                                    { type: 0, value: 5 },
+                                    { type: "term", value: 5 },
+                                    { type: "term", value: 5 },
                                 ],
                             },
                         ],
                     },
-                    { type: 0, value: 5 },
+                    { type: "term", value: 5 },
                 ],
             },
         ]);
@@ -203,87 +203,87 @@ describe("gp (mep)", () => {
             ast.decodeChromosome(ast.randomChromosome(), 3),
             [
                 {
-                    type: 1,
+                    type: "op",
                     op: "-",
                     args: [
-                        { type: 0, value: 5 },
+                        { type: "term", value: 5 },
                         {
-                            type: 1,
+                            type: "op",
                             op: "*",
                             args: [
-                                { type: 0, value: 5 },
-                                { type: 0, value: 5 },
+                                { type: "term", value: 5 },
+                                { type: "term", value: 5 },
                             ],
                         },
                     ],
                 },
                 {
-                    type: 1,
+                    type: "op",
                     op: "-",
                     args: [
                         {
-                            type: 1,
+                            type: "op",
                             op: "-",
                             args: [
-                                { type: 0, value: 5 },
+                                { type: "term", value: 5 },
                                 {
-                                    type: 1,
+                                    type: "op",
                                     op: "*",
                                     args: [
-                                        { type: 0, value: 5 },
-                                        { type: 0, value: 5 },
+                                        { type: "term", value: 5 },
+                                        { type: "term", value: 5 },
                                     ],
                                 },
                             ],
                         },
                         {
-                            type: 1,
+                            type: "op",
                             op: "-",
                             args: [
-                                { type: 0, value: 5 },
-                                { type: 0, value: 5 },
+                                { type: "term", value: 5 },
+                                { type: "term", value: 5 },
                             ],
                         },
                     ],
                 },
                 {
-                    type: 1,
+                    type: "op",
                     op: "*",
                     args: [
                         {
-                            type: 1,
+                            type: "op",
                             op: "-",
                             args: [
-                                { type: 0, value: 5 },
-                                { type: 0, value: 5 },
+                                { type: "term", value: 5 },
+                                { type: "term", value: 5 },
                             ],
                         },
-                        { type: 0, value: 5 },
+                        { type: "term", value: 5 },
                     ],
                 },
                 {
-                    type: 1,
+                    type: "op",
                     op: "-",
                     args: [
                         {
-                            type: 1,
+                            type: "op",
                             op: "*",
                             args: [
-                                { type: 0, value: 5 },
-                                { type: 0, value: 5 },
+                                { type: "term", value: 5 },
+                                { type: "term", value: 5 },
                             ],
                         },
                         {
-                            type: 1,
+                            type: "op",
                             op: "-",
                             args: [
-                                { type: 0, value: 5 },
+                                { type: "term", value: 5 },
                                 {
-                                    type: 1,
+                                    type: "op",
                                     op: "*",
                                     args: [
-                                        { type: 0, value: 5 },
-                                        { type: 0, value: 5 },
+                                        { type: "term", value: 5 },
+                                        { type: "term", value: 5 },
                                     ],
                                 },
                             ],
@@ -291,54 +291,54 @@ describe("gp (mep)", () => {
                     ],
                 },
                 {
-                    type: 1,
+                    type: "op",
                     op: "/",
                     args: [
-                        { type: 0, value: 5 },
+                        { type: "term", value: 5 },
                         {
-                            type: 1,
+                            type: "op",
                             op: "-",
                             args: [
-                                { type: 0, value: 5 },
-                                { type: 0, value: 5 },
+                                { type: "term", value: 5 },
+                                { type: "term", value: 5 },
                             ],
                         },
                     ],
                 },
                 {
-                    type: 1,
+                    type: "op",
                     op: "-",
                     args: [
                         {
-                            type: 1,
+                            type: "op",
                             op: "-",
                             args: [
                                 {
-                                    type: 1,
+                                    type: "op",
                                     op: "-",
                                     args: [
-                                        { type: 0, value: 5 },
+                                        { type: "term", value: 5 },
                                         {
-                                            type: 1,
+                                            type: "op",
                                             op: "*",
                                             args: [
-                                                { type: 0, value: 5 },
-                                                { type: 0, value: 5 },
+                                                { type: "term", value: 5 },
+                                                { type: "term", value: 5 },
                                             ],
                                         },
                                     ],
                                 },
                                 {
-                                    type: 1,
+                                    type: "op",
                                     op: "-",
                                     args: [
-                                        { type: 0, value: 5 },
-                                        { type: 0, value: 5 },
+                                        { type: "term", value: 5 },
+                                        { type: "term", value: 5 },
                                     ],
                                 },
                             ],
                         },
-                        { type: 0, value: 5 },
+                        { type: "term", value: 5 },
                     ],
                 },
             ]
@@ -347,16 +347,16 @@ describe("gp (mep)", () => {
 
     it("mutate", () => {
         assert.deepStrictEqual(ast.mutate(ast.randomChromosome()), [
-            { type: 0, value: 0 },
-            { type: 0, value: 5 },
-            { type: 1, op: "*", args: [1, 0] },
-            { type: 1, op: "+", args: [2, 1] },
-            { type: 1, op: "-", args: [0, 0] },
-            { type: 1, op: "+", args: [4, 2] },
-            { type: 1, op: "/", args: [3, 1] },
-            { type: 1, op: "/", args: [4, 3] },
-            { type: 0, value: 5 },
-            { type: 1, op: "/", args: [6, 4] },
+            { type: "term", value: 0 },
+            { type: "term", value: 5 },
+            { type: "op", op: "*", args: [1, 0] },
+            { type: "op", op: "+", args: [2, 1] },
+            { type: "op", op: "-", args: [0, 0] },
+            { type: "op", op: "+", args: [4, 2] },
+            { type: "op", op: "/", args: [3, 1] },
+            { type: "op", op: "/", args: [4, 3] },
+            { type: "term", value: 5 },
+            { type: "op", op: "/", args: [6, 4] },
         ]);
     });
 
@@ -364,43 +364,43 @@ describe("gp (mep)", () => {
         const a = ast.randomChromosome();
         const b = ast.randomChromosome();
         assert.deepStrictEqual(b, [
-            { type: 0, value: 5 },
-            { type: 1, op: "*", args: [0, 0] },
-            { type: 0, value: 5 },
-            { type: 1, op: "-", args: [0, 0] },
-            { type: 1, op: "/", args: [1, 0] },
-            { type: 1, op: "*", args: [2, 1] },
-            { type: 1, op: "-", args: [3, 2] },
-            { type: 1, op: "-", args: [2, 1] },
-            { type: 0, value: 1 },
-            { type: 0, value: 6 },
+            { type: "term", value: 5 },
+            { type: "op", op: "*", args: [0, 0] },
+            { type: "term", value: 5 },
+            { type: "op", op: "-", args: [0, 0] },
+            { type: "op", op: "/", args: [1, 0] },
+            { type: "op", op: "*", args: [2, 1] },
+            { type: "op", op: "-", args: [3, 2] },
+            { type: "op", op: "-", args: [2, 1] },
+            { type: "term", value: 1 },
+            { type: "term", value: 6 },
         ]);
         assert.deepStrictEqual(ast.crossoverSingle(a, b, 5), [
             [
-                { type: 0, value: 5 },
-                { type: 0, value: 5 },
-                { type: 1, op: "*", args: [1, 1] },
-                { type: 1, op: "-", args: [1, 2] },
-                { type: 1, op: "-", args: [0, 0] },
+                { type: "term", value: 5 },
+                { type: "term", value: 5 },
+                { type: "op", op: "*", args: [1, 1] },
+                { type: "op", op: "-", args: [1, 2] },
+                { type: "op", op: "-", args: [0, 0] },
                 // cut
-                { type: 1, op: "*", args: [2, 1] },
-                { type: 1, op: "-", args: [3, 2] },
-                { type: 1, op: "-", args: [2, 1] },
-                { type: 0, value: 1 },
-                { type: 0, value: 6 },
+                { type: "op", op: "*", args: [2, 1] },
+                { type: "op", op: "-", args: [3, 2] },
+                { type: "op", op: "-", args: [2, 1] },
+                { type: "term", value: 1 },
+                { type: "term", value: 6 },
             ],
             [
-                { type: 0, value: 5 },
-                { type: 1, op: "*", args: [0, 0] },
-                { type: 0, value: 5 },
-                { type: 1, op: "-", args: [0, 0] },
-                { type: 1, op: "/", args: [1, 0] },
+                { type: "term", value: 5 },
+                { type: "op", op: "*", args: [0, 0] },
+                { type: "term", value: 5 },
+                { type: "op", op: "-", args: [0, 0] },
+                { type: "op", op: "/", args: [1, 0] },
                 // cut
-                { type: 1, op: "-", args: [3, 4] },
-                { type: 1, op: "*", args: [4, 0] },
-                { type: 1, op: "-", args: [2, 3] },
-                { type: 1, op: "/", args: [1, 4] },
-                { type: 1, op: "-", args: [5, 0] },
+                { type: "op", op: "-", args: [3, 4] },
+                { type: "op", op: "*", args: [4, 0] },
+                { type: "op", op: "-", args: [2, 3] },
+                { type: "op", op: "/", args: [1, 4] },
+                { type: "op", op: "-", args: [5, 0] },
             ],
         ]);
     });
@@ -409,16 +409,16 @@ describe("gp (mep)", () => {
         const a = ast.randomChromosome();
         const b = ast.randomChromosome();
         assert.deepStrictEqual(ast.crossoverUniform(a, b), [
-            { type: 0, value: 5 },
-            { type: 1, op: "*", args: [0, 0] },
-            { type: 0, value: 5 },
-            { type: 1, op: "-", args: [1, 2] },
-            { type: 1, op: "-", args: [0, 0] },
-            { type: 1, op: "-", args: [3, 4] },
-            { type: 1, op: "-", args: [3, 2] },
-            { type: 1, op: "-", args: [2, 1] },
-            { type: 0, value: 1 },
-            { type: 1, op: "-", args: [5, 0] },
+            { type: "term", value: 5 },
+            { type: "op", op: "*", args: [0, 0] },
+            { type: "term", value: 5 },
+            { type: "op", op: "-", args: [1, 2] },
+            { type: "op", op: "-", args: [0, 0] },
+            { type: "op", op: "-", args: [3, 4] },
+            { type: "op", op: "-", args: [3, 2] },
+            { type: "op", op: "-", args: [2, 1] },
+            { type: "term", value: 1 },
+            { type: "op", op: "-", args: [5, 0] },
         ]);
     });
 });