% A version of 'reverse' which requires 'removeLast' and 'last':
reverse([],[]).
reverse([F|R],L2) :- last(L2,F), removeLast(L2,L2less), reverse(R,L2less).

%  removeLast, Last -- not included in notes, 
%  since it's part of hw05  !!



%% Alternate version: An accumulator variable:
reverse(L1,L2) :- reverseHelp( L1, [], L2 ).

%%  [+,+,-]
%% reverseHelp(L1,L2,L3): 
%% True when: L1 reversed, appended to L2, equals L3.
%    First, a less idiomatic solution:
% reverseHelp([],RevSoFar,Soln) :- RevSoFar = Soln.
% reverseHelp(Lst, RevSoFar, Soln) :-
%    Lst = [Frst,Rst],
%    reverseHelp( Rst, [Frst,RevSoFar], Soln ).

% Here is the same thing, in idiomatic Prolog:
% We just move the "=" clauses into the definitions:
%
reverseHelp([], RevSoFar, RevSoFar).
reverseHelp([Frst|Rst], RevSoFar, Out) :- 
    reverseHelp(Rst, [Frst|RevSoFar], Out).


%% [+,+,-]
%% filterSmallerThan( Threshold, Lst, Result ):
%% True when 'Result' is all the elements of 'Lst' which
%% are less than Threshold.

%    First, a less idiomatic solution:
% filterLessThan(Threshold,Lst,Answer) :- Lst=[], Answer=[].
% filterLessThan(Threshold,Lst,Answer) :- 
%     Lst = [Frst,Rst],
%     Frst >= Threshold, 
%     filterLessThan(Threshold,Rst,SmallRest),
%     Answer = SmallRest.
% filterLessThan(Threshold,Lst,Answer) :- 
%     Lst = [Frst,Rst],
%     Frst < Threshold, 
%     filterLessThan(Threshold,Rst,SmallRest),
%     Answer = [Frst|SmallRest].

% Here is the same thing, in idiomatic Prolog:
% We just move the "=" clauses into the definitions:
%
filterLessThan(Threshold,[],[]).
filterLessThan(Threshold,[Frst|Rst],SmallRest) :- 
      Frst >= Threshold, 
      filterLessThan(Threshold,Rst,SmallRest).
filterLessThan(Threshold,[Frst|Rst],[F|SmallRest]) :- 
      Frst < Threshold, 
      filterLessThan(Threshold,Rst,SmallRest).