code: public int size() {
  	int size = 0;
  	for(K key : keys()) {
  	    size += getValues(key).size();
  	}
  	return size;
      }
docs: Returns the number of &lt;key,value&gt; pairs in
  <code>this</code> relation.  Linear in the size of the
  relation.  This may be also implemented in O(1) by
  incrementally updating a <code>size</code> field, but that may
  complicate the code in the presence of subclassing, etc.  Will
  think about it if it becomes a problem.