better proof end

[kohlhase]

Currently, \end{sproof} makes a \Box unless we redefine it via proofend=. The consequence is that the box is always on a new line, which counter mathematical practice. It should be inside the last \spfstep.

I understand that this is difficult to do. So I would be willing to specify the last step, e.g.

\begin{sproof}
...
\spfstep[last]{the last step comes with a proofend}
\end{sproof} % if there was a last step, then this does not make one. 

Maybe this is simpler.