generate enumerate all subsets of {O,●，O} (x0+x2)(ax0+x2)(x0+x2) =2x020+x0xx1+20x'x0+20zlxI +x1x0x0+x1a0x1+x1x1a0+1xx1 (1+x)3=1+3a+3x2+x3 coefficient ofx:of k-subsets { , , } enumerate all subsets of generate (x0 + x1)(x0 + x1)(x0 + x1) = + + + + + + + x1 x0x0x0 x0x0x1 x0 x0 x1 x0 x1 x1 x0 x0 x1 x0 x1 x1 x0 x1 x1 x1 x1 (1 + x) 3 =1+3x + 3x2 + x3 coefficient of xk : # of k-subsets