@Override
        public Builder addRegistry(final IoRegistry registry) {
            throw new UnsupportedOperationException("GraphML does not accept custom serializers - it is a format for full Graph serialization");
        }
This is an "empty" implementation only present for compatibility with {@link Io}.  GraphML is a "whole graph"
serialization format and does not have the notion of generic serialization functions.

@author Stephen Mallette (http://stephen.genoprime.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/structure/io/graphml/GraphMLMapper.java