集合A
{abc}
{abd}
{abe}
{abf}
{abcd}
{abce}
{abcf}
{abcde}
{abcdef}
{abdef}
{abdf}
{abef}
.就是集合A一定有元素{a b}
可能有{cdef}一个或2个或3个或4个元素