prolog - Appending a list to a list of lists recursively -

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the problem i'm trying solve follows:

i'm given sorted list must pair first , last items in list. must pair 2nd , (last-1) items in list until list either empty or 1 element remains. must return list of pairs.

the steps decided take problem first check if list's length greater 1. if wasn't, means have list of 0 or 1 elements.

then first , last items in given list, delete them list, pair them, , recursively call same predicate on new list. once i've gone way down 0/1 items, pop , append them return list.

the problem i'm having when try append pair l = [first,last] return list, errors out. code listed below.

t input list.

first/2 gets first item in list. pair/3 strips away info p1 , p2 , creates l = [p1,p2]. 

getmatches(t,k,returnlist) :-    (  length(t,val),       val > 1,        first(t,p1),        last(t, p2),       delete(t,p1,g),       delete(g,p2,h),       pair(p1,p2,l),       getmatches(h,k,returnlist),       append(l,k,returnlist)    ;  first(t,_),       k = []    ).

an example use: if t = [1, 2, 3, 4, 5] returnlist = [[1,5], [2, 4]] should hold.

getmatches(list, returnlist) :-        % getmatches/2     getmatches(list, [], answer),     reverse(answer, returnlist),     !.  getmatches(list, listans, listans) :-  % getmatches/3     length(list, l),     l < 2. getmatches([h | tail], list, ans) :-     last(tail, last),     delete(tail, last, newtail),     append([[h, last]], list, newlist),     getmatches(newtail, newlist, ans).

and

?- getmatches([1,2,3,4,5],x). x = [[1, 5], [2, 4]].

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