Loop on undefined values, refs #2721

Author: JJ

doc/Language/control.pod6

@@ -235,7 +235,7 @@ for it more strongly:
     $_ = 1; if 42 -> $a { $_.say;  $a.say } ; # says "1" then says "42"
     $_ = 1; if 42       { $_.say; $^a.say } ; # says "1" then says "42"
 
-=head2 X<else/elsif|control flow,else elsif>
+=head2 X<C<else/elsif>|control flow,else elsif>
 
 A compound conditional may be produced by following an C<if> conditional
 with C<else> to provide an alternative block to run when the conditional
@@ -312,7 +312,7 @@ present:
 
     if False { } elsif 0 { } else -> $a { $a.say }  ; # says "0"
 
-=head2 X<unless|control flow, unless>
+=head2 X<C<unless>|control flow, unless>
 
 When you get sick of typing "if not (X)" you may use C<unless> to invert
 the sense of a conditional statement.  You cannot use C<else> or C<elsif>
@@ -524,6 +524,17 @@ say "Not here" for Empty;
 Will not do anything. This constant is
 L<equivalent to a empty Slip or List|/syntax/Empty>.
 
+Undefined values will behave in the same way:
+
+=for code
+my @array := Empty;
+.say for @array;
+say @array; # OUTPUT: «()␤»
+
+Assigning C<Empty> will effectively undefine an Array, using C<for> over an
+undefined array will not even enter the loop, as shown, effectively behaving in
+the same way as above when C<Empty> was used directly.
+
 =head1 X<gather/take|control flow,gather take>
 
 X<|lazy list,gather>X<|lazy list,take>