reserve T for non empty RelStr,
  A,B for Subset of T,
  x,x2,y,z for Element of T;

theorem Th20:
  Fint(A,1) = A^i
proof
  (Fint A).0=A & for B being Subset of T st B=(Fint A).0 holds (Fint A).(0
  +1)= B^i by Def4;
  hence thesis;
end;
