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