@SuppressWarnings("unchecked")
        private K wrapKey(Object o) {
            if (o==null) return null;
            return (K) keyTypeVespa.createFieldValue(o);
        }
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