feat(hott): finish cancelling law for sums with unit
feat(hott): finish cancelling law for sums with unit

leanprover/lean,Kha/lean,javra/lean,soonhokong/lean-osx,johoelzl/lean,leodemoura/lean,johoelzl/lean,soonhokong/lean,fpvandoorn/lean2,fgdorais/lean,leodemoura/lean,digama0/lean,levnach/lean,avigad/lean,leodemoura/lean,leanprover/lean,leanprover-community/lean,johoelzl/lean,leodemoura/lean,dselsam/lean,Kha/lean,soonhokong/lean-windows,levnach/lean,soonhokong/lean-osx,dselsam/lean,soonhokong/lean-osx,rlewis1988/lean,fpvandoorn/lean,soonhokong/lean-windows,fpvandoorn/lean,leanprover/lean,fpvandoorn/lean,digama0/lean,fpvandoorn/lean2,digama0/lean,rlewis1988/lean,soonhokong/lean,johoelzl/lean,johoelzl/lean,avigad/lean,leodemoura/lean,soonhokong/lean-osx,UlrikBuchholtz/lean,UlrikBuchholtz/lean,dselsam/lean,fgdorais/lean,Kha/lean,UlrikBuchholtz/lean,fgdorais/lean,levnach/lean,dselsam/lean,javra/lean,soonhokong/lean,UlrikBuchholtz/lean,javra/lean,levnach/lean,soonhokong/lean-windows,javra/lean,leanprover-community/lean,leanprover/lean,Kha/lean,soonhokong/lean-osx,fgdorais/lean,soonhokong/lean-windows,fpvandoorn/lean,soonhokong/lean-windows,dselsam/lean,dselsam/lean,rlewis1988/lean,rlewis1988/lean,levnach/lean,Kha/lean,digama0/lean,leodemoura/lean,digama0/lean,fgdorais/lean,fpvandoorn/lean,UlrikBuchholtz/lean,fpvandoorn/lean2,digama0/lean,leanprover/lean,fpvandoorn/lean2,leanprover-community/lean,leanprover/lean,soonhokong/lean,leanprover-community/lean,rlewis1988/lean,avigad/lean,rlewis1988/lean,johoelzl/lean,javra/lean,avigad/lean,fgdorais/lean,avigad/lean,Kha/lean,soonhokong/lean,avigad/lean,leanprover-community/lean,fpvandoorn/lean2,fpvandoorn/lean,leanprover-community/lean