% If you have your prolog rules in file "filename.pl", to load it in the
% prolog interpreter, key in :
% prolog -l filename.pl

% +,? are just symbolic, not to be used in implementing predicates

% Prolog has in-built predicates number() and var() for checking numbers and variables repectively

% Note that the following functions can be implemented in different and better ways


% The car function in prolog
car([X|L],X).

% The cdr function in prolog
cdr([X|L],L).

% A function that pushes X onto a list as num(X)
% Example invokation : push(1,[2,3,4], Output).
	 
push(X,L,[num(X)|L]).


% A function that returns the first "number" on the input list

firstnumber([X|L],X):-number(X).
firstnumber([X|L],Y):- firstnumber(L,Y).

% A function that returns a list of all the numbers contained in the input list
% Example invokation : numbers([1,2,3,a,b,c,+], [], Output). 

numbers([], Accum, Accum).
numbers([X|L], Accum, Output):- number(X), numbers(L, [Accum|X], Output).
numbers([X|L], Accum, Output) :- numbers(L, Accum, Output).


% A function that returns two lists, one containing all the operators and the other containing all non-operators contained in the input list
% Example invokation : r(Output1, Output2, [], [], [1,2,3,a,b,+,-,a]).

r(Ops, Args, Ops, Args, []).
r(Output1, Output2, Ops, Args, [Top|Stack]):- operator(Top), r(Output1, Output2, [Top|Ops], Args, Stack).
r(Output1, Output2, Ops, Args, [Top|Stack]):- r(Output1, Output2, Ops, [Top|Args], Stack).