public static <T> void incr(final Map<T, Long> map, final T key, final Long value) {
        map.put(key, value + map.getOrDefault(key, 0l));
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/util/MapHelper.java