replace 'prompt' example by 'environment'

lib/structures/dict.js

@@ -18,6 +18,13 @@ export const set = dict => key => value => new RBTreeWrapper(dict.tree.remove(ke
 export const remove = dict => key => new RBTreeWrapper(dict.tree.remove(key), inspectDict);
 export const length = dict => dict.tree.length;
 
+export const fold = callback => initial => dict => {
+  let acc = initial;
+  for (const iter=dict.tree.begin; iter !== undefined && iter.valid; iter.next()) {
+    acc = callback(acc)(iter.key)(iter.value);
+  }
+}
+
 export const first = dict => dict.tree.begin;
 export const last  = dict => dict.tree.end;