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 Th16:
  a,b @@ c,d & a,b @@ c,d9 implies d = d9
by Th8;
