fix(library/tactic/goal): bug creating main proof builder
fix(library/tactic/goal): bug creating main proof builder

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

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