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;
