\newenvironment{bnf}%
    {\begin{grammar}
        [(colon){ $\longrightarrow$\ \ignorespaces}]
        [(semicolon){\penalty10000 \ \hskip -20pt plus 1 fil \penalty0
                     \hbox{}\hskip 20pt plus -1fil $|$\ \ignorespaces}]
        [(period){\\}][(nonterminal){$\langle$\bgroup\tt}{\egroup$\rangle$}]}%
    {\end{grammar}}