// script to check normal orientation in freefem
load"gmsh"
mesh Th=gmshload("cylinder.msh");
fespace Wh(Th,P0);
Wh checker;
varf check(u,v)=int1d(Th,5)(v*(N.x<1e-10 && abs(N.y)<1e-10));
checker[]=check(0,Wh);
plot(checker,fill=1,wait=1,value=1);