What does the double minus (--) convention in function names mean in Emacs Lisp

I've been reading through a number of Emacs Lisp packages开发者_StackOverflow社区 and have come across the convention of some functions being declared with -- after the library prefix, e.g.:

(defun eproject--combine-regexps (regexp-list)

I'm wondering if this a convention for declaring "private" functions to the library but so far I haven't found anything in the Emacs Coding guidelines.

Emacs doesn't have any support for namespaces, packages, libraries or modules. Emacs sources therefore use foo- as a prefix for a foo library, and in some cases foo-- is used for bindings that are supposed to be internal.

There is really no such thing as "internal" for Emacs. But yes, some programmers have adopted this convention to indicate things that are more internal -- meaning essentially that there will be less (or no) hesitation by implementors to change them. It's a way of letting users of the code be aware of this possible volatility.