make $x++ return 0 on appropriate undefs

Since the operator is doing something defined, the first
value it returns should also be defined.

Author: TimToady

S03-operators.pod

@@ -15,8 +15,8 @@ Synopsis 3: Perl 6 Operators
 
     Created: 8 Mar 2004
 
-    Last Modified: 23 Sep 2010
-    Version: 220
+    Last Modified: 10 Dec 2010
+    Version: 221
 
 =head1 Overview
 
@@ -543,6 +543,12 @@ previously incremented.  This is useful if your C<%seen> hash is
 actually a C<KeySet>, in which case decrement actually deletes it
 from the C<KeySet>.
 
+Increment/decrement of an undefined C<Numeric>, C<Cool>, or C<Any> variable
+sets the variable to 0 and then performs the increment/decrement.  Hence a
+postincrement returns 0 the first time:
+
+    my $x; say $x++;    # 0, not Any
+
 =over
 
 =item *