[plt-scheme] proof checking: a subproblem

> (define simple-lexer
>   (lexer
>     ;; skip whitespace
>     (scheme-whitespace (simple-lexer input-port))
>     (identifier (token-ID lexeme))
>     ("&" 'AND)
>     ("|" 'OR)
>     ("(" 'LPAREN)
>     (")" 'LPAREN)
            ^^^^^^

Gah.  My apologies about that; I meant to type out RPAREN.  I knew I
should have tested this on a larger example.