theorem Th22:
  a1,a2,a3 are_collinear implies a1,a3,a2 are_collinear & a2,a1,a3
  are_collinear & a2,a3,a1 are_collinear & a3,a1,a2 are_collinear & a3,a2,a1
  are_collinear
by Th7;
