reserve SAS for Semi_Affine_Space;
reserve a,a9,a1,a2,a3,a4,b,b9,c,c9,d,d9,d1,d2,o,p,p1,p2,q,r,r1,r2,s,x, y,t,z
  for Element of SAS;

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;
