I have recently created a coding language called Valyrio. I have also posted a proposal for a question in the Sandbox. I have a problem though. It has been well-established that any builtin or language created after the code-golf has been posted is not allowed and any answer after this is non-competing. However, I added a builtin to Valyrio after I posted the Sandbox proposal that can do this challenge. The question, as of yet, has not been posted as a code-golf problem. Does this mean I can use the built in to complete the challenge or does it violate the non-competing rule?
(But based on what I've seen of your built in, probably not)
Here's where the ambiguity comes in. If your built in makes your answer boring, you probably shouldn't post it (or you could include it in another answer of the same language, If you felt the need). If your built in makes your answer unfair (but not uninteresting!), you may want to post it as it's own answer, but noncompeting.
Or you could just leave it out, especially if your answer would only be a few bytes.
From (briefly) reading the documentation, It looks like your answer would be very short due to the built in. In that case, I would not post it, as it most likely falls under both categories.
The bottom line is, If your answer is nontrivial and interesting, then go for it. Otherwise, just let it pass.