code: function insideDefine(rule) {
      var parent = rule.parent;
      if(!parent) {
          return false;
      } else if(parent.name === DEFINEFUNCTION_KEY) {
          return true;
      } else {
          return insideDefine(parent);
      }
  }
docs: refer to : https://github.com/postcss/postcss-mixins/blob/master/index.js
  beacuse last version code is too ugly, I decide to refactor my codes.