@Override
    public Set<String> getScopeKeys() {
        if (this.expression.getVariables().contains(CURRENT)) {
            final Set<String> temp = new HashSet<>(this.expression.getVariables());
            temp.remove(CURRENT);
            return temp;
        } else
            return this.expression.getVariables();
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/map/MathStep.java