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 Th4:
  a,b // c,d implies b,a // c,d
proof
  assume
A1: a,b // c,d;
  a,b // b,a by Def1;
  then a<>b implies b,a // c,d by A1,Def1;
  hence thesis by A1;
end;
