🚀 go-pugleaf

Here's some code I don't understand.

append([],L,L) :- !.
append([X|L1],L2,[X|L3]) :- append(L1,L2,L3). 

next_to(El1,El2,List) :- append(_,[El1,El2|_], List).

In the second edition of Clocksin and Mellish, p. 150, they state that the
above definition of "next_to" is an example of relational programming.  By
typing:

next_to(a,b,[c,d,a,b,q]).

for example, you'll get a true response.  I don't know why it works,
probably because I never could quite understand how append works.  Since
I'm missing an important concept here, could someone explain to me (and the
net) why it works?  The debugger in the Prolog interpreter (Amziod) I have
doesn't seem to go into enough detail to show me exactly what's happening.

Thanks,

Art
-- 
Art Winterbauer  N0OQS
Internet:  awinterb@du.edu OR awinterb@diana.cair.du.edu
Packet:    n0oqs @ w0gvt.#neco.co.usa