public static <T, U> void incr(final Map<T, List<U>> map, final T key, final U value) {
        map.compute(key, (k, v) -> {
            if (null == v) v = new ArrayList<>();
            v.add(value);
            return v;
        });
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/util/MapHelper.java