Let $S=\{a,b,c,d,e,f\}$. In how many ways can one select two subsets (not necessarily distinct) of $S$ so that their union is $S$? Note:The order of selection does not matter. For example, $\{a,c,e\},\{b,c,d,f\}$ and $\{b,c,d,f\},\{a,c,e\}$ are counted as same selection.