code: private Variable getVariableAtNode(int index, CallStack callstack) throws UtilEvalError {
          Node nameNode = null;
          if (jjtGetChild(index).jjtGetNumChildren() > 0
                  && (nameNode = jjtGetChild(index).jjtGetChild(0))
                      instanceof BSHAmbiguousName)
              return callstack.top().getVariableImpl(
                      ((BSHAmbiguousName) nameNode).text, true);
          return null;
      }
docs: Get Variable for value at specified index.
  @param index 0 for lhs val1 else 1
  @param callstack the evaluation call stack
  @return the variable in call stack name space for the ambiguous node text
  @throws UtilEvalError thrown by getVariableImpl.