theorem Th44:
  not a,b,c are_collinear implies ex d st parallelogram a,b,c,d
proof
  assume
A1: not a,b,c are_collinear;
  consider d such that
A2: a,b // c,d & a,c // b,d by Def1;
  take d;
  thus thesis by A1,A2;
end;
