A bit further on it says:

> For example, given a specific length, we can ask whether there are lists of that length:

    ?- list_length(Ls, 3).
    Ls = [_G1007, _G1087, _G1167] ;
    false.

In the current situation, if you ask for `list_length(Ls, -1)`, this will fail, obviously because there are no lists with length -1. But if you remove the `N #> 0` clause, the code will happily try to find such a list, by checking for lists with length -2, -3, ... ad infinitum.

Another way to look at this is that if you have a list `[_|Ls]` with length N, its length must be an integer greater than 0.