code: public static Rule getRule(final Stack stack) {
      if (stack.size() < 1) {
        throw new IllegalArgumentException(
          "Invalid EXISTS rule - expected one parameter but received "
          + stack.size());
      }
  
      return new ExistsRule(stack.pop().toString());
    }
docs: Create an instance of ExistsRule using the
  top name on the stack.
  @param stack stack
  @return instance of ExistsRule.