reserve SAS for Semi_Affine_Space;
reserve a,a9,a1,a2,a3,a4,b,b9,c,c9,d,d9,d1,d2,o,p,p1,p2,q,r,r1,r2,s,x, y,t,z
  for Element of SAS;

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;
