theorem Th87:
  ^(0,T,I) = \0(T,I)
  proof
    ex f being Function of INT, (the Sorts of T).I st
    ^(0,T,I) = f.0 & f.0 = \0(T,I) &
    for j being Nat, t being Element of T,I st f.j = t
    holds f.(j+1) = t+\1(T,I) & f.(-(j+1)) = -(t+\1(T,I)) by Def15;
    hence thesis;
  end;
