reserve x,y,z for Real,
  x3,y3 for Real,
  p for Point of TOP-REAL 3;
reserve p1,p2,p3,p4 for Point of TOP-REAL 3,
  x1,x2,y1,y2,z1,z2 for Real;

theorem
  |[x1, y1, z1]| <X> |[x2, y2, z2]| =
    |[ (y1 * z2) - (z1 * y2), (z1 * x2) - (x1 * z2), (x1 * y2) - (y1 * x2) ]|;
