Use A(AN) when talking about a thing which is new, unknown, or introduced to a listener for the first time.
At the same time, they say
The definite article is used when the speaker talks about a specific object.
Now let's look at the following sentence. This is a commit message for the version control system I'm trying to write correctly:
Added a/the/Ø prototype for a/the/Ø bar() function
On the one hand, I mention this function and its prototype for the first time, but on the other hand, I talk about specific objects. So which articles should I use?