Exercises

1. When you use the difference list format, it is important to be careful with the choice of variable names. What would happen if you made the following query?

   | ?- List = [a,b,c|X], X = [d,e,f|X].
   

2. We do not strictly use the difference list format to concatenate lists, but rather to identify parts of lists. You could use it to concatenate lists however. Here is a single fact which encodes difference list concatenation:

   dl_conc(S0-S1,S1-S2,S0-S2).
   

   (I have used A-B instead of A,B to make it easier to see which lists belong together.) Call it with a few different lists so that you understand how it works.
3. To think about: Why is an explicit call to dl_conc not needed in the Prolog procedure?