\begin{pf}
Left as an exercise to the grader.
\end{pf}

\begin{pf}[this should override heading]
\end{pf}

\begin{pf}[this should override heading]{this should be ignored}
i dont want a lot for christmas

all i want for christmas is this proof
\end{pf}

\begin{pf}{sp4n1sh_1nqu151t10n}
\end{pf}
