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:];
reserve X,Y,X1,X2 for non empty set,
          cA1,cB1 for filter_base of X1,
          cA2,cB2 for filter_base of X2,
              cF1 for Filter of X1,
              cF2 for Filter of X2,
             cBa1 for basis of cF1,
             cBa2 for basis of cF2;

theorem Th25:
  [n,n] in square-uparrow n
  proof
    n in NAT by ORDINAL1:def 12;
    then reconsider x = [n,n] as Element of [:NAT,NAT:] by ZFMISC_1:def 2;
    x`1 = n & x`2 = n;
    hence thesis by Def3;
  end;
