@Override
        public Set<K> keySet() {
            Set<K> ret = new HashSet<>();
            for (Map.Entry e : map.entrySet()) {
                ret.add(wrapKey(e.getKey()));
            }
            return ret;
        }
Weighted set MapFieldValue, backed by map of native Java types.
Note: The key type of this is FieldValue, not K.
@author vegardh
document/src/main/java/com/yahoo/document/datatypes/WeightedSet.java