Let S be the set of bit strings defined recursively by S and 0x

Let S be the set of bit strings defined recursively by λ ∈ S and 0x ∈ S, x1 ∈ S if x ∈ S, where λ is the empty string.
a) Find all strings in S of length not exceeding five.
b) Give an explicit description of the elements of S.