public Set<String> getVariables() {
            return variables;
        }
A wrapper for the {@code Expression} class. That class is not marked {@code Serializable} and therefore gives
problems in OLAP specifically with Spark. This wrapper allows the {@code Expression} to be serialized in that
context with Java serialization.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/map/MathStep.java