Odd prolog syntax

gistfile1.pl

% Following code sample from 'Higher Order Logic Programming In Prolog' by Lee Naish
% www.csee.umbc.edu/771/papers/mu_96_02.pdf

% insertion sort (simple version)
isort([], []).
isort(A.As, Bs) :-
    isort(As, Bs1),
    isort(A, Bs1, Bs).

% insert number into sorted list
insert(N, [], [N]).
insert(N, H.L, N.H.L) :-
    N =< H.
insert(N, H.LO, H.L) :-
    N > H,
    insert(N, LO, L).