tests: Check for inverse semigroup property in OrgMode parser
tests: Check for inverse semigroup property in OrgMode parser

Ever since [1] we allow a second representation for the month (namely,
the numerical one).  Since we lose this information during parsing,
pretty printing is now not a proper postinverse of parsing (it still is
a proper preinverse, however).

Thus, we can't simply check for an inverse anymore.  However, the
operations still form an inverse semigroup [2], which is something
that's easily checkable.  For simplicity, do this in both directions and
completely forget about linearity for now.

[1]: 91f1a0de1e4a536e6c81a3de96e566622b3eb20a (Fix date parsing issue
                                               for org mode plugin)
[2]: https://en.wikipedia.org/wiki/Inverse_semigroup

xmonad/xmonad-contrib,xmonad/xmonad-contrib