check for collinearity
bool coll = true ; coll = coll & collinear ( p1 , p2 , p3 ) ; coll = coll & collinear ( p1 , p2 , p4 ) ; coll = coll & collinear ( p1 , p3 , p4 ) ; coll = coll & collinear ( p2 , p3 , p4 ) ;