theorem
  a 'nor' (a 'imp' b) = O_el(Y)
proof
  thus a 'nor' (a 'imp' b) = 'not' a '&' a '&' 'not' b by Th36
    .= O_el(Y) '&' 'not' b by BVFUNC_4:5
    .= O_el(Y) by BVFUNC_1:5;
end;
