🚀 go-pugleaf

In article <5091.25e18bfa@mva.cs.liv.ac.uk>, duncan@mva.cs.liv.ac.uk writes:
> 
>   The problem is this. Using this definition,  define append/4 which will
> append 3 lists to form a 4th *AND* will work reversibly to generate all
> sublist solutions without looping on backtracking.
> 

Here is my solution ...

append([],L,L).
append([H|T],L,[H|T1]) :- append(T,L,T1).

app4([],[],L,L).
app4([],[H|T],L1,[H|T1]) :- append(T,L1,T1).
app4([H|T],L1,L2,[H|T1]) :- app4(T,L1,L2,T1).

First, narrow it down to two lists, then use regular append.
Also works in generation of sublists recursively. Note that 
on the second clause, an [H|T] notation was used for the second
argument in order to disable binding to an empty list and
therefore duplicating results on sublists.

Boris Pelakh				"Software - a spell one casts on a
pelakh@cerc.wvu.wvnet.edu		 computer to transform input into
coop@cerc.wvu.wvnet.edu			 errors."        -- Me

Disclaimer :    If my employer knew what I did with the time I get paid for,
		I would be out of a job. Let's keep this between us, OK ?