theorem Th65:
  Collinear b,a,x & Collinear c,d,x & b <> x & c <> x implies
  a,b,c,d are_coplanar
  proof
    assume Collinear b,a,x & Collinear c,d,x & b <> x & c <> x;
    then b,a,c,d are_coplanar by Th64;
    hence thesis;
  end;
