private static void FixSegmentDirection(seg3d s1, Face f1, Face f2, bool reverse)
{
xyz n1 = f1.UnitNormal();
xyz n2 = f2.UnitNormal();
xyz k = xyz.cross(n2, n1).normalize_in_place();
if (reverse)
{
k.negate_in_place();
}
xyz vab = (s1.b - s1.a).normalize_in_place();
if (!fp.eq_unitvec(vab, k))
{
xyz t = s1.a;
s1.a = s1.b;
s1.b = t;
}
}