写题解的时候应该用哪个？
pi&pj=Xp_i \mathbin{\&} p_j = Xpi&pj=X $p_i \mathbin{\&} p_j = X$
$p_i \mathbin{\&} p_j = X$
pi&pj=Xp_i \& p_j = Xpi&pj=X $p_i \& p_j = X$
$p_i \& p_j = X$
