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sub pack (@array) returns Array {
    my @unpacked = @array;
    my (@list, @packed);
...
    return @packed;
}
pack(<a a a a b c c a a d e e e e>).perl.say;

(defun pack (lista)
    (if (eql lista nil) 
        nil
...
          (t (cdr lista))
    )
)

pack([],[]).
pack([X|Xs],[Z|Zs]) :- transfer(X,Xs,Ys,Z), pack(Ys,Zs).

transfer(X,[],[],[X]).
transfer(X,[Y|Ys],[Y|Ys],[X]) :- X \= Y.
transfer(X,[X|Xs],Ys,[X|Zs]) :- transfer(X,Xs,Ys,Zs).

group (x:xs) = let (first,rest) = span (==x) xs
               in (x:first) : group rest
group [] = []