.nud     Null denotation .fud     First null denotation .led     Left denotation lbp     Left binding power rbp     Right binding power They are elements of the parsing method called Top Down Operator Precedence.
function expression(rbp, initial) {
        var left;
        if (nexttoken.id === '(end)') {
            error("Unexpected early end of program.", token);
        }
        advance();
        if (option.safe && typeof predefined[token.value] === 'boolean' &&
                (nexttoken.id !== '(' && nexttoken.id !== '.')) {
            warning('ADsafe violation.', token);
        }
        if (initial) {
            anonname = 'anonymous';
            funct['(verb)'] = token.value;
        }
        if (initial === true && token.fud) {
            left = token.fud();
        } else {
            if (token.nud) {
                left = token.nud();
            } else {
                if (nexttoken.type === '(number)' && token.id === '.') {
                    warning(
"A leading decimal point can be confused with a dot: '.{a}'.",
                            token, nexttoken.value);
                    advance();
                    return token;
                } else {
                    error("Expected an identifier and instead saw '{a}'.",
                            token, token.id);
                }
            }
            while (rbp < nexttoken.lbp) {
                advance();
                if (token.led) {
                    left = token.led(left);
                } else {
                    error("Expected an operator and instead saw '{a}'.",
                        token, token.id);
                }
            }
        }
        return left;
    }