theorem Th12:
  ex a,b,c,d st not a,b,c,d are_coplanar
proof
  consider a,b,c,d such that
A1: not a,b,x are_collinear or not c,d,x are_collinear by ANPROJ_2:def 14;
  take a,b,c,d;
  thus thesis by A1;
end;
