Write a Lisp program that generates a list containing all legal successor states for any state in the Missionaries and Cannibals problem. Your program should be invoked by the function call:
where state is of the form (m,n,b), with m representing the number of missionaries (0-3) on the left bank, n representing the number of cannibals (0-3) on the left bank, and b representing whether the boat is on the right side (r) or the left side (I) of the river.
(mac-next (33I)) => ((22r) (32r) (31r))
(mac-next (31r)) => ((32I) (33l))
(mac-next (33r)) => nil
Your function should check whether the inputted state is in the proper format. If not, an error message should be displayed instead of the list of successor states.