theorem Th26:
  I_el(Y) 'nor' a = O_el(Y)
proof
  thus I_el(Y) 'nor' a = 'not' (I_el(Y) 'or' a) by Th2
    .= 'not' I_el(Y) by BVFUNC_1:10
    .= O_el(Y) by BVFUNC_1:2;
end;
