simplify: no distinction between generic types and 'normal' types.

lib/util/util.js

@@ -25,3 +25,15 @@ const _mapRecursiveStructure = mapping => transform => root => {
 };
 
 export const mapRecursiveStructure = _mapRecursiveStructure(new Map());
+
+export const memoize = callback => {
+  let called = false
+  let result;
+  return () => {
+    if (!called) {
+      result = callback();
+      called = true;
+    }
+    return result;
+  };
+};