restructure code a bit, add comparison functions for primitive types and composed types (needed to put values in sets)

structures/set.js

@@ -24,6 +24,20 @@ export const emptySet = compareFn => new RBTreeWrapper(createRBTree((x, y) => co
 export const has = set => key => set.tree.get(key) === true;
 export const add = set => key => set.tree.get(key) === true ? set : new RBTreeWrapper(set.tree.insert(key, true));
 export const remove = set => key => new RBTreeWrapper(set.tree.remove(key));
+export const length = set => set.tree.length;
+
+export const first = set => set.tree.begin;
+export const last  = set => set.tree.end;
+
+// test if iterator is 'done', and if not, get element and advance iterator.
+export const read = iter => ifNotDone => ifDone => {
+  if (iter !== undefined && iter.valid) {
+    return ifNotDone(iter.key)(iter.clone().next());
+  }
+  else {
+    return ifDone();
+  }
+};
 
 export const forEach = set => fn => {
   set.tree.forEach(key => { fn(key); });