[z5h]

For example: the logical core of Prolog along with it's resolution model (for the logical part) are non deterministic (something can have none, one, many solutions) but only one solution is explored at a time. So it's a "meta logical" thing to express something like "the set of solutions for ...". Given that the core of Prolog is Turing complete, you can still get Prolog to compute anything, you might just not have a nice way of declaring it in pure Prolog.

Prolog has an interesting history of people discovering ways to express things that are simple, powerful and elegant. And yet despite the simplicity, these ways of expressing things were not immediately evident. DCGs are a prime example.