code: public static final double det(final Point2D p, final Point2D q)
      {
          return (p.getX() * q.getY()) - (p.getY() * q.getX());
      }
docs: Returns the determinant of vectors P and Q. By definition:
  <ul>
  <li>det(P, Q) = -det(Q, P)
  <li>det(P, -Q) = -det(P, Q)
  <li>if P == alpha * Q   &lt;=&gt;  det(P, Q) = 0 (P and Q on same line)
  <li>det(P, Origin) = 0
  <li>OQ parallel OP  &lt;=&gt;  det(P,Q) = 0
  </ul>
  Imagine standing on the Origin, facing point P:
  <ul>
  <li>if Q is on your left, det(P,Q) &gt; 0
  <li>if Q is on your right, det(P,Q) &lt; 0
  <li>if Q is on the line thru P and the Origin, det(P,Q) = 0
  </ul>
  
  @param p Point2D
  @param q Point2D
  @return the determinant of vectors P and Q