% ex.pl
% some example predicates that may help with assignment # 4

% remove an element (which much be present) from a list...
removeOne(X,[X|R],R).
removeOne(X,[F|R],[F|S]) :- removeOne(X,R,S).

% generates permutations of the first input's list
perm([X|Y],Z) :- perm(Y,W), removeOne(X,Z,W).
perm([],[]).

% checks if the input list is sorted
sorted( [] ).   % zero-element lists are sorted.
sorted( [_] ).  % one-element lists are sorted
sorted( [F,S|R] ) :- sorted( [S|R] ), F < S.

% binds the second argument to a sorted version of the first
mysort( [], [] ).
mysort( L, S ) :- perm( L, S ), sorted(S).  % inefficient!