Suppose that in designing a gas pipeline network, we have 2 possible pipe sizes, small ( S ) and large ( L ). If there are 4 possible links, enumerate all possible pipeline networks. (A typical one could be abbreviated LSLL , where the i th letter indicates the size of the i th pipe.)
SSSSSSSLSSLSSSLLSLSSSLSLSLLSSLLLLSSSLSSLLSLSLSLLLLSSLLSLLLLSLLLL