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# DCG problem

## DCG problem

(OP)
I have to write a DCG that generates strings from the alphabet a E {0,1,2} the strings have to have the form nn. This means queries like ?- s([0,2,1,0,2,1],[]). and s([1,1,2,0,1,1,2,0],[]). would return true.

This is the code I have so far. I'm so close. Can someone help me further?

s --> s(X), s(X).
s --> [].

s(0) --> zero, s(X).
s(1) --> one,  s(X).
s(2) --> two,  s(X).
s(3) --> [].

zero --> [0].
one  --> [1].
two  --> [2].

Many thanks ;)

s --> [].
s --> sx, s.

sx --> [0].
sx --> [1].
sx --> [2].

### RE: DCG problem

You are very close to the solution !

Just look at these rules
s --> s(X), s(X).
and
s(0) --> zero, s(X).

In the first rule, what is the type of X, an atom or a list ?
In the second rule you use an atom 0, and you introduce X in the body of the rule. You don't have a lot of things to change/add.

### RE: DCG problem

No kahleen  the production must have the form nn, you accept [0,1,2] for example.

### RE: DCG problem

(OP)
Great! Thanks joel! I got it!

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