Get elements from list of lists

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You asked for all elements of a list of lists. That is, for [[1,2,3],[4]] this would be the list [1,2,3,4]. However, for [[[1],[3]]] this would be the list [[1],[3]] since [1] and [3] are elements. For this reason, flatten/2 is incorrect it gives you [1,3] as an answer. Also, for 1 it gives [1]…

Here is a solution using dcg:

seq([]) --> [].
seq([E|Es]) --> [E], seq(Es).

seqq([]) --> [].
seqq([Es|Ess]) --> seq(Es), seqq(Ess).

?- phrase(seqq([[[1],[3]]]), Xs).
   Xs = [[1],[3]].
?- phrase(seqq(1), Xs).
   false.

This solution now works also for cases like the following:

?- phrase(seqq([S1,S2]), [1,2]).
   S1 = [], S2 = [1,2]
;  S1 = [1], S2 = [2]
;  S1 = [1,2], S2 = []
;  false.

Whereas flatten/2 is completely wrong:

?- flatten([S1,S2],[1,2]).
   S1 = 1, S2 = 2.

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