Prolog list of "lists" with only one element

Question

I have a recursive predicate call, and the output of that call is appended to a list with append/3. So the output is going to be a list of lists. My problem is, that sometimes the append/3 gets called something like this:

append([[2]], [1,2,3,4], L).

And the output of this is [[2], 1,2,3,4]. I would like to check before the append/3 that the first element is a list of lists with only one element, but so far I was unable to create a pattern that matches it.

So my question is: how can I check if something is a list of a list?

Answer 1

The rule that matches a Prolog atom is, well, atom/1:

is_list_of_list_with_one_element([[X]]) :- atom(X).