Fix bug in is_convertible
Fix bug in is_convertible

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

fpvandoorn/lean,soonhokong/lean-windows,levnach/lean,c-cube/lean,fpvandoorn/lean2,rlewis1988/lean,eigengrau/lean,fpvandoorn/lean,avigad/lean,johoelzl/lean,fpvandoorn/lean,htzh/lean,htzh/lean,avigad/lean,c-cube/lean,dselsam/lean,c-cube/lean,javra/lean,fgdorais/lean,soonhokong/travis_test,fpvandoorn/lean2,Kha/lean,leodemoura/lean,leanprover/lean,UlrikBuchholtz/lean,soonhokong/lean-windows,soonhokong/travis_test,fpvandoorn/lean2,htzh/lean,Kha/lean,avigad/lean,digama0/lean,fgdorais/lean,soonhokong/lean,leodemoura/lean,fgdorais/lean,leanprover-community/lean,levnach/lean,fpvandoorn/lean,soonhokong/lean-osx,sp3ctum/lean,soonhokong/travis_test,fgdorais/lean,rlewis1988/lean,eigengrau/lean,leanprover/lean,UlrikBuchholtz/lean,leodemoura/lean,codyroux/lean0.1,Kha/lean,soonhokong/lean,rlewis1988/lean,soonhokong/lean-windows,javra/lean,codyroux/lean0.1,rlewis1988/lean,soonhokong/lean-windows,leanprover-community/lean,digama0/lean,fpvandoorn/lean2,htzh/lean,codyroux/lean0.1,soonhokong/lean-osx,javra/lean,soonhokong/lean,Kha/lean,soonhokong/lean,johoelzl/lean,leanprover-community/lean,leanprover/lean,soonhokong/lean-osx,digama0/lean,levnach/lean,fgdorais/lean,avigad/lean,sp3ctum/lean,fpvandoorn/lean,johoelzl/lean,dselsam/lean,codyroux/lean0.1,c-cube/lean,eigengrau/lean,javra/lean,leanprover-community/lean,leanprover/lean,leodemoura/lean,soonhokong/travis_test,avigad/lean,fpvandoorn/lean,Kha/lean,digama0/lean,UlrikBuchholtz/lean,dselsam/lean,dselsam/lean,sp3ctum/lean,johoelzl/lean,rlewis1988/lean,levnach/lean,soonhokong/lean-osx,johoelzl/lean,leodemoura/lean,leodemoura/lean,javra/lean,Kha/lean,avigad/lean,leanprover/lean,UlrikBuchholtz/lean,digama0/lean,htzh/lean,c-cube/lean,levnach/lean,digama0/lean,soonhokong/lean,sp3ctum/lean,UlrikBuchholtz/lean,dselsam/lean,leanprover-community/lean,soonhokong/lean-windows,johoelzl/lean,soonhokong/lean-osx,leanprover/lean,leanprover-community/lean,eigengrau/lean,sp3ctum/lean,eigengrau/lean,dselsam/lean,rlewis1988/lean,fpvandoorn/lean2,fgdorais/lean