fix(util/sexpr): Lua 5.1 incompatibility
fix(util/sexpr): Lua 5.1 incompatibility

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

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