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:];
reserve u,v,w,u9,w9 for Vector of M;

theorem
  vect(a,b) = vect(a,c) implies b = c
proof
  assume vect(a,b) = vect(a,c);
  then a,b @@ a,b & a,b @@ a,c by Th20,Th28;
  hence thesis by Th16;
end;
