code: function replaceFn(name, args, content) {
      var newContent = '';
      var oneVarRE = /(\$\w+)/g;
      var variableRE = /(\$\w+)([\+\-\*/@])/g;
      var content = removeSpace(content);
      // to get '$a' in the end of a content string beacuse of my bad Reg
      content = content + '@';
      var length = args.length;
  
      if (length) {
          if(length === 1) {
              content = content.replace(oneVarRE, args[0]);
          } else {
              var i = 0;
              var replacer = function(str, $1, $2) {
                  if(args[i]){
                      return args[i++] + $2;
                  } else {
                      return str;
                  }
  
              }
              content = content.replace(variableRE, replacer);
          }
  
      } else {
          // no params
          content = content;
      }
      // delete '@'
      newContent = content.substr(0, content.length - 1);
  
  
  
      return newContent;
  }
docs: [replaceFn description]
  @param  {[type]} name    [custom function name]
  @param  {[type]} args    [custom function arguments]
  @param  {[type]} content [custom function content]
  @return {[String]}
  
  
  rem(640) => 640 / 640 * 10 rem;