code: function normalizeFunctionSignature(signature, callback) {
    if (!/^\*|@warn|@error|@debug|\w+\(.*\)$/.test(signature)) {
      if (!/\w+/.test(signature)) {
        throw new Error('Invalid function signature format "' + signature + '"');
      }
  
      return {
        signature: signature + '(...)',
        callback: function() {
          var args = Array.prototype.slice.call(arguments),
            list = args.shift(),
            i;
  
          for (i = list.getLength() - 1; i >= 0; i--) {
            args.unshift(list.getValue(i));
          }
  
          return callback.apply(this, args);
        }
      };
    }
  
    return {
      signature: signature,
      callback: callback
    };
  }
docs: Normalizes the signature of custom functions to make it possible to just supply the
  function name and have the signature default to `fn(...)`. The callback is adjusted
  to transform the input sass list into discrete arguments.
  
  @param {String} signature
  @param {Function} callback
  @return {Object}
  @api private