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 Th43:
  parallelogram a,b,c,d implies parallelogram a,c,b,d &
  parallelogram c,d,a,b & parallelogram b,a,d,c & parallelogram c,a,d,b &
  parallelogram d,b,c,a & parallelogram b,d,a,c
by Th38,Th6;
