reserve Y for non empty set;

theorem
  for a,b,c being Function of Y,BOOLEAN holds ((a 'imp' b) 'imp' c
  ) 'imp' (b 'imp' c)=I_el(Y)
proof
  let a,b,c be Function of Y,BOOLEAN;
  b 'imp' (a 'imp' b) =I_el(Y) by Th15;
  hence thesis by Th14;
end;
