Combinators combine elements together, in a Selector.  Because our parser isn't white-space sensitive, special care has to be taken, when parsing the descendant combinator, ` `, as it's an empty space. We have to check the previous character in the input, to see if it's a ` ` character. More info on how we deal with this in *combinator.js*.
function () {
                var c = parserInput.currentChar();

                if (c === '/') {
                    parserInput.save();
                    var slashedCombinator = parserInput.$re(/^\/[a-z]+\//i);
                    if (slashedCombinator) {
                        parserInput.forget();
                        return new(tree.Combinator)(slashedCombinator);
                    }
                    parserInput.restore();
                }

                if (c === '>' || c === '+' || c === '~' || c === '|' || c === '^') {
                    parserInput.i++;
                    if (c === '^' && parserInput.currentChar() === '^') {
                        c = '^^';
                        parserInput.i++;
                    }
                    while (parserInput.isWhitespace()) { parserInput.i++; }
                    return new(tree.Combinator)(c);
                } else if (parserInput.isWhitespace(-1)) {
                    return new(tree.Combinator)(' ');
                } else {
                    return new(tree.Combinator)(null);
                }
            }