alpha) and return it as a lowercase
letter. If C is instantiated and the DCG list is already bound,
C must be lower and matches both a lower and uppercase letter.
If the output list is unbound, its first element is bound to C.
For example:
?- alpha_to_lower(0'a, `AB`, R). R = [66]. ?- alpha_to_lower(C, `AB`, R). C = 97, R = [66]. ?- alpha_to_lower(0'a, L, R). L = [97|R].