private E untilNonNull(final Iterator<E> itty) {
        E result = null;
        while (itty.hasNext() && null == result) {
            result = itty.next();
        }
        return result;
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
@author Daniel Kuppitz (http://gremlin.guru)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/map/SumLocalStep.java