diff --git a/src/main.ts b/src/main.ts
index 79af291d..c23b00aa 100644
--- a/src/main.ts
+++ b/src/main.ts
@@ -11,7 +11,7 @@ import { namedTypes } from "./gen/namedTypes";
 import { builders } from "./gen/builders";
 import { Visitor } from "./gen/visitor";
 
-const {
+let {
   astNodesAreEquivalent,
   builders,
   builtInTypes,
@@ -45,6 +45,27 @@ const {
 // retaining its member types.
 Object.assign(namedTypes, n);
 
+export function overrideDefs(defs: ReturnType<typeof fork>) {
+  astNodesAreEquivalent = defs.astNodesAreEquivalent;
+  builders = defs.builders;
+  builtInTypes = defs.builtInTypes;
+  defineMethod = defs.defineMethod;
+  eachField = defs.eachField;
+  finalize = defs.finalize;
+  getBuilderName = defs.getBuilderName;
+  getFieldNames = defs.getFieldNames;
+  getFieldValue = defs.getFieldValue;
+  getSupertypeNames = defs.getSupertypeNames;
+  Object.assign(namedTypes, defs.namedTypes);
+  NodePath = defs.NodePath;
+  Path = defs.Path;
+  PathVisitor = defs.PathVisitor;
+  someField = defs.someField;
+  Type = defs.Type;
+  use = defs.use;
+  visit = defs.visit;
+}
+
 export {
   AnyType,
   ASTNode,