def PerpProduct2D(a,b):
    """Computes the the 2D perpendicular product of sequences a and b.
       The convention is a perp b.
       The product is:
          positive if b is to the left of a
          negative if b is to the right of a
          zero if b is colinear with a
       left right defined as shortest angle (< 180)
    """
    return (a[0] * b[1] - a[1] * b[0])