code: function calc(token, addTokenToMatch, getNextToken) {
      if (token === null) {
          return false;
      }
  
      var name = token.value.toLowerCase();
      if (name !== 'calc(' &&
          name !== '-moz-calc(' &&
          name !== '-webkit-calc(') {
          return false;
      }
  
      return consumeFunction(token, addTokenToMatch, getNextToken);
  }
docs: TODO: implement can be used wherever <length>, <frequency>, <angle>, <time>, <percentage>, <number>, or <integer> values are allowed https://drafts.csswg.org/css-values/#calc-notation