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 Th27:
  square-uparrow n = [:NAT \ Segm n, NAT \ Segm n:]
  proof
    reconsider no = n as Element of OrderedNAT by ORDINAL1:def 12;
    uparrow no = NAT \Segm n by Th13;
    hence thesis by Th26;
  end;
