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;
reserve p,q,r,p9,q9 for Element of [:the carrier of M,the carrier of M:];

theorem Th29:
  [a,b]~ = [c,d]~ implies a@d = b@c
proof
  assume [a,b]~ = [c,d]~;
  then a,b @@ c,d by Th20,Th28;
  hence thesis;
end;
