rule assignment {
        <primary> '=' <expression>
        {*}
    }

    rule primary {
        <identifier>
        {*}
    }

    token identifier {
        <!keyword> <ident>
        {*}
   }

    token keyword {
        ['and'|'catch'|'do'   |'else' |'end' |'for' |'if'
        |'not'|'or'   |'sub'  |'throw'|'try' |'var'|'while']>>
    }