sum([ ], 0).
sum([H|T], R) :- number(H), sum(T, Q), R is H+Q.
sum([H|T], R) :- not(number(H)), sum(T, R).

% That's the cut operator, which basically states "if we got up to here, don't backtrack."
% So, if both recursive calls succeed, the hi predicate will not retry them

% ! "cut" the search

sum2([ ], 0).
sum2([H|T], R) :- number(H), !, sum2(T, Q), R is H+Q.
sum2([H|T], R) :- sum2(T, R).

app([ ], L, L).
app([H|T], L, [H|Z]) :- app(T, L, Z).


rev([ ], [ ]).
rev([H|T], L) :- rev(T, Q), app(Q, [H], L).


flat([ ], [ ]).
flat(X, [X]) :- atomic(X), not(X=[ ]).
flat([H|T], R) :- flat(H, X), flat(T, Y), app(X, Y, R).


% call - param 1 function, param 2 var to exeucte function with, param 3 var to give result to

map(F, [ ], [ ]).
map(F, [H|T], [X|Y]) :- call(F, H, X), map(F, T, Y).

not(X) :- \+ X.