subscripts also map Nil --> Nil

This used to fall out of the fact that subscripts were methods, but
they aren't anymore, so we have to make this explicit.

Author: TimToady

S02-bits.pod

@@ -13,8 +13,8 @@ Synopsis 2: Bits and Pieces
 
     Created: 10 Aug 2004
 
-    Last Modified: 2 Oct 2013
-    Version: 278
+    Last Modified: 28 Dec 2013
+    Version: 279
 
 This document summarizes Apocalypse 2, which covers small-scale
 lexical items and typological issues.  (These Synopses also contain
@@ -507,7 +507,7 @@ C<Nil.gist> return C<'Nil'>.  C<Nil.Stringy> and C<Nil.Str> throw a resumable wa
 that returns a value of C<''> on resumption.  C<Nil.Numeric> likewise throws
 a resumable warning that returns 0 on resumption.  Any undefined method call
 on C<Nil> returns C<Nil>, so that C<Nil> propagates down method
-call chains.
+call chains.  Likewise any subscripting operation on C<Nil> returns C<Nil>.
 
 Assigning C<Nil> to any scalar container causes the container to
 throw out any contents and restore itself to an uninitialized state