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

lib/structures/set.js

@@ -11,6 +11,15 @@ export const add = set => key => set.tree.get(key) === true ? set : new RBTreeWr
 export const remove = set => key => new RBTreeWrapper(set.tree.remove(key));
 export const length = set => set.tree.length;
 
+export const fold = set => callback => initial => {
+  let acc = initial;
+  let iter = set.tree.begin;
+  while (iter !== undefined && iter.valid) {
+    acc = callback(acc, iter.key);
+  }
+  return acc;
+};
+
 export const first = set => set.tree.begin;
 export const last  = set => set.tree.end;