public static VariableResolver instance() {
            return instance;
        }
This {@link VariableResolver} simply provides a {@code null} value for all variable names. It's typical use
is for when you really don't intend to execute the traversal and just want to get an instance of one when
bindings are being used as with {@link NoOpTerminalVisitor}.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/language/grammar/VariableResolver.java