Couple of weird things about Prolog: It's a pattern-matcher, in much the same way Haskell is -- but Prolog invented it first. Thus the first rule tries to match the args you give it, and if you give it the literals [] and 0, it just says "True" or "Yes" (depending on which Prolog you're using). If you give it something it cannot match to [] and 0, it will try the second clause and assume the first arg is a list with a head and a tail, and the second arg is an integer.

The other weird thing is that Prolog functions can often run backwards. If you ask Prolog

?- list_length(Foo, 4)

it will hand you back a list of 4 elements that it just made up on the spot. This is why Prolog rules are considered generalizations of functions: They don't have explicit inputs vs. outputs. They're relations, not functions.

This stuff blew my mind when I first encountered Prolog and it changed forever how I thought about programming.