fix(library/unifier): bug exposed by recent changes
fix(library/unifier): bug exposed by recent changes

Signed-off-by: Leonardo de Moura <7610bae85f2b530654cc716772f1fe653373e892@microsoft.com>

javra/lean,sp3ctum/lean,htzh/lean,johoelzl/lean,UlrikBuchholtz/lean,soonhokong/lean-windows,leanprover/lean,Kha/lean,htzh/lean,rlewis1988/lean,avigad/lean,c-cube/lean,javra/lean,digama0/lean,javra/lean,fpvandoorn/lean,leanprover/lean,rlewis1988/lean,leanprover-community/lean,leodemoura/lean,sp3ctum/lean,dselsam/lean,soonhokong/lean,c-cube/lean,avigad/lean,soonhokong/lean-windows,fpvandoorn/lean2,dselsam/lean,leanprover-community/lean,johoelzl/lean,avigad/lean,fgdorais/lean,johoelzl/lean,rlewis1988/lean,htzh/lean,johoelzl/lean,Kha/lean,avigad/lean,UlrikBuchholtz/lean,levnach/lean,dselsam/lean,rlewis1988/lean,soonhokong/lean-osx,soonhokong/lean-windows,eigengrau/lean,dselsam/lean,eigengrau/lean,eigengrau/lean,c-cube/lean,fgdorais/lean,c-cube/lean,soonhokong/lean-windows,leanprover-community/lean,digama0/lean,sp3ctum/lean,levnach/lean,c-cube/lean,fpvandoorn/lean,sp3ctum/lean,soonhokong/lean-osx,leodemoura/lean,UlrikBuchholtz/lean,leanprover/lean,htzh/lean,soonhokong/lean,soonhokong/lean-osx,fpvandoorn/lean,johoelzl/lean,soonhokong/lean-osx,dselsam/lean,sp3ctum/lean,rlewis1988/lean,eigengrau/lean,leanprover-community/lean,leodemoura/lean,Kha/lean,UlrikBuchholtz/lean,soonhokong/lean,eigengrau/lean,soonhokong/lean-osx,leanprover/lean,fpvandoorn/lean2,Kha/lean,digama0/lean,fpvandoorn/lean,dselsam/lean,avigad/lean,fpvandoorn/lean2,leanprover-community/lean,Kha/lean,fpvandoorn/lean2,fgdorais/lean,levnach/lean,Kha/lean,soonhokong/lean,rlewis1988/lean,soonhokong/lean-windows,levnach/lean,fpvandoorn/lean,fpvandoorn/lean,leanprover/lean,leanprover-community/lean,htzh/lean,levnach/lean,avigad/lean,leanprover/lean,digama0/lean,fgdorais/lean,leodemoura/lean,UlrikBuchholtz/lean,johoelzl/lean,digama0/lean,digama0/lean,javra/lean,fgdorais/lean,leodemoura/lean,soonhokong/lean,leodemoura/lean,fpvandoorn/lean2,fgdorais/lean,javra/lean