@Override
        public Object apply(final String s, final GremlinParser.VariableContext variableContext) {
            throw new VariableResolverException(String.format("No variable found for %s", s));
        }
This {@link VariableResolver} implementation throws exceptions for all variable names.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/language/grammar/VariableResolver.java