reserve Y for non empty set,
  a,b,c,d for Function of Y,BOOLEAN;

theorem
  a 'or' (a 'imp' b) = I_el(Y)
proof
  a 'or' (a 'imp' b) =a 'or' ('not' a 'or' b) by BVFUNC_4:8
    .=(a 'or' 'not' a) 'or' b by BVFUNC_1:8
    .=I_el(Y) 'or' b by BVFUNC_4:6;
  hence thesis by BVFUNC_1:10;
end;
