Add Library constants

diff --git a/inflibqr/inflibqr.md b/inflibqr/inflibqr.md
index a5eaf7d36bc13e5ddc3ab6f3cce23e6809bd6c5b..67fdb4e8a262d6b9b0a488a471491864e2fa8b65 100644 (file)
--- a/inflibqr/inflibqr.md
+++ b/inflibqr/inflibqr.md
@@ -40,4 +40,11 @@ Library objects
 variable, which usually refers to `selfobj`.
 
 `thedark` - A pseudo-room which becomes the `location` when there is
-no light (although the player object is not moved there).
\ No newline at end of file
+no light (although the player object is not moved there).
+
+Library constants
+-----------------
+
+In addition to the standard constants `true` (1), `false` (0) and
+`nothing` (0), the Library defines `NULL` (-1) for an action, property
+or pronoun whose current value is undefined.
\ No newline at end of file