A,B,C,D

the ways are-CDAB,CBAD,DABC,BADC

permutations is a generator; each time it is called it will return a new permutation of the sequence with which it was constructed. The for loop arranges to call this generator until it is exhausted. concatenated is the result of one iteration of the generator concatenated together to form a string; this is done to make for convenient processing in subsequent statements. The first if statement arranges to ignore a permutation if it begins or ends with 'A'. The second one ignores a permutation if it contains either 'AC' or 'CA'. If a permutation passes both tests then it is counted.

We can arrange either this way: _ A C or this way: C A__ where B and D are free to arrange themselves in the blank spaces. In each of the 2 cases, we have 2! = 2 ways. So the total is $2 \times 2=\boxed{4}$