docs: function () {
                  var match, c = input.charAt(i);
  
                  if (c === '>' || c === '+' || c === '~') {
                      i++;
                      while (input.charAt(i).match(/\s/)) { i++ }
                      return new(tree.Combinator)(c);
                  } else if (input.charAt(i - 1).match(/\s/)) {
                      return new(tree.Combinator)(" ");
                  } else {
                      return new(tree.Combinator)(null);
                  }
              }