reserve            x for object,
               X,Y,Z for set,
         i,j,k,l,m,n for Nat,
                 r,s for Real,
                  no for Element of OrderedNAT,
                   A for Subset of [:NAT,NAT:];

theorem Th4:
  x in [:X,Y:] implies x is pair
  proof
    assume x in [:X,Y:];
    then ex a,b be object st a in X & b in Y & x = [a,b] by ZFMISC_1:def 2;
    hence thesis;
  end;
