TBD: Should matching an "EMPTY rule" succeed?

[xparq]

First of all, what exactly an "empty rule" is, is pretty much undefined:

• there isn't really an empty rule like {}, as it gets autoconverted to NIL. Actually: {NIL}, even...
• should it be "" instead?! And should that be the same as {""}, too?...
• should NIL and "" mean the same, so therefore match the same way?
• then how about "" always matching (unanchored), while NIL was supposed to be a forced failed match (as opposed to the forced successful match of T)?!...

---

Since every rule is boolean, it's an unavoidable question...

• If false (NIL), then hitting an empty rule could cancel the matching of the entire syntax, rather than just being redundant, as one might expect, and as is with the _EMPTY pattern!...

• OTOH, in LISP, () is false (NIL).

• Currently it's inconsistent, too: using "" would create a NIL rule, but using _EMPTY would match anything successfully.
  _Not sure which one is right... _EMPTY might actually be wanted for matching the empty string exactly, and not matching a non-empty one... So, it's ambiguous.

  • Maybe the distinction is "explicit" empty (_EMPTY) vs "implicit" empty ("")?
  • NO: The distinction is that match() by default works with substrings! So _EMPTY duly matches the empty substring of everything!

• Anyway, there should be a straightforward and reliable way to choose to match the empty string only (in the sense that any non-empty string should not match). That should be achievable with anchoring (-> Add _END for anchoring, to reject extraneous input after a full match #21).

• It also feels natural to have both a forced-false (NIL) and a forced-true (T) rule, so this really is a conundrum now... The empty Production kinda must be NIL (to mimic LISP), but then it would not match anything, which would be inconsistent with the empty Atom ("") matching everything...

  • Maybe it's time to recognize and separate the result of matching something from the "result" -- the intrinsic value -- of the rule that's being matched! So, matching NIL (or false, for that matter) against "anything" can still be true! But should it?... Still don't know...