11. Let a and b be two characters. Define the set of strings S over these two characters recursively as follows: s in a 1S in s bs in is in S if u and v are v1 in A is called the empty-strina: it is a string with no characters. uv is called the concatenation of strings u and v. For example, if u is the string aba and v is the string bbaa, then uv is the string ababbaa. We define several functions. (A) = 0 (b) = 1 So l(x) is the length of the string x. fa(A) fa (a) 0 =