theorem Th34:
  not a,b,c are_collinear implies ex d st parallelogram a,b,c,d
proof
  consider d such that
A1: a,b '||' c,d & a,c '||' b,d by PARSP_1:def 12;
  assume not a,b,c are_collinear;
  then parallelogram a,b,c,d by A1;
  hence thesis;
end;
