Help with Exercise in DCG chapter

[razetime]

I am trying to define palindrome in terms of reversal, and this is what I got:

reversal([]) --> [].
reversal([L|Ls]) --> reversal(Ls), [L].

palindrome --> [].
palindrome --> [_].
palindrome --> [E], palindrome, [E].

pal1 --> seq(X), reversal(X).
pal1 --> seqq([X,[_]]), reversal(X).

However, in the most general form I get the following results for pal1:

   X = []
;  X = [_A,_A]
;  X = [_A,_B,_B,_A]
;  X = [_A,_B,_C,_C,_B,_A]
;  X = [_A,_B,_C,_D,_D,_C,_B,_A]
;  ... .

This is too specific from what I can garner, but I'm not entirely sure how I can factor this into a proper solution.