theorem
  a 'nor' 'not' a = O_el(Y) & 'not' (a 'nor' 'not' a) = I_el(Y)
proof
  a 'nor' 'not' a = 'not' (a 'or' 'not' a) by Th2
    .= 'not' I_el(Y) by BVFUNC_4:6
    .= O_el(Y) by BVFUNC_1:2;
  hence thesis by BVFUNC_1:2;
end;
