Add lemma `StronglySorted_app_iff`
This merge request adds the lemma StronglySorted_app_iff (and the corollary StronglySorted_app), and rewrites the existing *_app lemmas as a corollary of StronglySorted_app_iff.
Edited by Marijn van Wezel
This merge request adds the lemma StronglySorted_app_iff (and the corollary StronglySorted_app), and rewrites the existing *_app lemmas as a corollary of StronglySorted_app_iff.