A function call  rgb(255, 0, 255)  The arguments are parsed with the `entities.arguments` parser.
function () {
                    var name, args, func, index = parserInput.i;

                    // http://jsperf.com/case-insensitive-regex-vs-strtolower-then-regex/18
                    if (parserInput.peek(/^url\(/i)) {
                        return;
                    }

                    parserInput.save();

                    name = parserInput.$re(/^([\w-]+|%|progid:[\w\.]+)\(/);
                    if (!name) {
                        parserInput.forget(); 
                        return;
                    }

                    name = name[1];
                    func = this.customFuncCall(name);
                    if (func) {
                        args = func.parse();
                        if (args && func.stop) {
                            parserInput.forget();
                            return args;
                        }
                    }

                    args = this.arguments(args);

                    if (!parserInput.$char(')')) {
                        parserInput.restore('Could not parse call arguments or missing \')\'');
                        return;
                    }

                    parserInput.forget();
                    
                    return new(tree.Call)(name, args, index, fileInfo);
                }