reserve MS for non empty MidStr;
reserve a, b for Element of MS;
reserve M for MidSp;
reserve a,b,c,d,a9,b9,c9,d9,x,y,x9 for Element of M;

theorem Th5:
  (a@b)@c = (a@c)@(b@c)
proof
  (a@b)@c = (a@b)@(c@c) by Def3
    .= (a@c)@(b@c) by Def3;
  hence thesis;
end;
