Variables changing for no apparent reason!

Hi, First post on this board, apologies if I haven't adhered to any etiquett
e rules, and hope that somebody can help me!

I've got a problem that I just don't understand. I'm used to other programmi
ng languages, but not Prolog.
I am writing a program to navigate a path between several nodes in any order
.

my problem is that I have a list containing what it should contain, and then
 I use another function (which doesn't use this list at all), and then strai
ght after this, the list contains something else!

headTowards is a recursive function, which moves from one node to another (a
 journey). This works fine when run on its own.

singleJourney is also recursive.  Each iteration should move from the curren
t position to a node from TargetList, and then remove that node from the lis
t (storing the new list in NewTargetList and using that as the new TargetLis
t when singleJourney is called again).

Code listing and output below.

Thanks in advance,
Ben

code:

singleJourney(BoardSize, [], StartPos, Moves, Moves, N, NTot):-
  write('GOT HERE!'), nl,
  NTot is N.

singleJourney(BoardSize, TargetList, StartPos, Moves, NewMoves, N, NTot):-
  member(Next, TargetList),
  del(Next, TargetList, NewTargetList),
  write('List at A: '), write(NewTargetList), nl,
  headTowards(BoardSize, StartPos, Next, Moves, B, 0, NMoves),
  write('List at B: '), write(NewTargetList), nl,
  N1 is N + NMoves,
  N1 < 6,
  singleJourney(BoardSize, NewTargetList, Next, B, NewMoves, N1, NTot).




headTowards(BoardSize, Current, Current, Moves, NewMoves, N, N1):-
  N1 is 0.

headTowards(BoardSize, Current, Target, Moves, NewMoves, N, NTot):-
  singleMove(BoardSize, Current, Next),
  not(member(Next, Moves)),
  N1 is N+1,
  N1 < 7,
  Moves1 = [Next | Moves],
  headTowards(BoardSize, Next, Target, Moves1, NewMoves, N1, NTot).



Example output from the debug writes:

List at A: []
List at B: [ (5, 3)]


This then repeats a lot, because at point B, the list isn't empty, so the en
d case is never called.

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