fix(library/coercion): bug in de-Bruijn indices
fix(library/coercion): bug in de-Bruijn indices

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

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