Triangle_dt next_t(Point_dt pp1, Point_dt pp2, Triangle_dt curr,
List<Triangle_dt> tr)
{
Triangle_dt ans = null, t12, t23, t31;
t12 = curr.next_12();
t23 = curr.next_23();
t31 = curr.next_31();
if (t12 != null && cut(pp1, pp2, t12) && !tr.Contains(t12))
ans = t12;
else if (t23 != null && cut(pp1, pp2, t23) && !tr.Contains(t23))
ans = t23;
else if (t31 != null && cut(pp1, pp2, t31) && !tr.Contains(t31))
ans = t31;
return ans;
}
