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 Th28:
  not a,b,c are_collinear & a,b // c,d & c <>d & c,d,x are_collinear
  implies not a,b,x are_collinear
by Th8,Th27;
