* Returns proof Pi (currently including line above and below Pi), with numerals n, i, i+1: (n + n) + i+1 = n + (n + i+1), Ax |- ...
* Returns proof Pi (currently including line above and below Pi), with numerals n, i, i+1: (n + n) + i+1 = n + (n + i+1), Ax |- ... Pi (n + n) + i = n + (n + i), Ax |- ...
Close off proof ending in (n + n) + 0 = n + (n + 0) |- ...
Takes a proof s2 with end-sequent of the form (x=y), ...
Takes a proof s2 with end-sequent of the form (x=y), ... |- ... and return one with end-sequent of the form (y=x), ... |- ...