-(0) = 0   

latex

Definitions	IMonoid, t  T, IGroup, t.2, t.1, , P & Q, Mon, Group{i}, AbGrp, <+>, e, ~, |g|, x:A. B(x)
Lemmas	abgrp wf, grp inv wf, inverse wf, grp id wf, grp op wf, grp car wf, monoid p wf, qadd grp wf, grp inv id