Create a recursive function ALPHABET that takes a list of letters. It returns a list of the same length with the corresponding integer substituted for the letter (e.g. A becomes 1). Do error checking on the input. Return the empty list if the list is already empty. You may use a global parameter to store letter-integer pairs.

[42]>  (alphabet '(a b z)) (1 2 26) [43]>  (alphabet '())  NIL [44]>  (alphabet "foo")  Error in ALPHABET function. Parameter must be a list, but was given "foo" NIL