public default GraphTraversal<S, ?> incrementProperty(final String propertyKey,
        final GraphTraversal<?,?> valueTraversal) {
    return property(Cardinality.single, propertyKey,
            __.union(
                __.values(propertyKey),
                valueTraversal)
            .sum());
}

public default GraphTraversal<S, ?> decrementProperty(final String propertyKey,
        final GraphTraversal<?,?> valueTraversal) {
    return property(Cardinality.single, propertyKey,
            __.union(
                __.values(propertyKey),
                valueTraversal).math("_ * -1")
            .sum());
}