The general parsing routine needed is:

[ FrenchNumber n;
    switch(NextWord())
    {   'un', 'une': n=1;
        'deux': n=2;
        'trois': n=3;
        'quatre': n=4;
        'cinq': n=5;
        default: return -1;
    }
    parsed_number = n; return 1;
];