Re: read-file-name-predicate

[Richard Stallman]

    `read-file-name-predicate' is a Lisp variable. It's doc says only that it is
    the current predicate used by built-in function `read-file-name-internal'.
    This is the only doc about it, and it gives the impression that it is only
    an internal variable for a built-in function. If that were true, then why
    would it be a Lisp variable instead of a C variable?

Sometimes a Lisp variable is more convenient.  For instance, it works
naturally with the specbind mechanism.

      `read-file-name-predicate' has an external,
    Lisp behavior: During completion, Lisp code can check what the predicate is
    or even change the predicate. Shouldn't that be documented?

I see no reason to document it at present.
It is meant for internal use only.