%% Case 1: the input list is empty. %% So, the output list should be empty as well. add_one(,). %% Case 2: the input list is not empty. %% The head of the output list is obtained by adding 1 to the head of %% the input list. The tails of the input and the output lists are %% again lists, and the ouput tail is the list that is obtained from %% adding 1 to each element of the input tail. That is, %% add_one(InTail,OutTail) should be true. add_one([Head|Tail],[OutHead|OutTail]) :- OutHead is Head + 1, add_one(Tail,OutTail).