theorem Th66:
  Collinear a,b,x & Collinear c,d,x & b <> x & c <> x implies
  a,b,c,d are_coplanar
  proof
    assume Collinear a,b,x & Collinear c,d,x & b <> x & c <> x;
    then Collinear b,a,x & Collinear c,d,x & b <> x & c <> x by GTARSKI3:14;
    hence thesis by Th65;
  end;
