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.
Currently,
\end{sproof}makes a\Boxunless we redefine it viaproofend=. 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.
Maybe this is simpler.