Document Nil → Any trap closes #1134

Uses also #1135 example to document it.

Author: JJ

doc/Language/traps.pod6

@@ -43,6 +43,41 @@ a behaviour similar to a constant, but allowing the value to get updated:
 unit module Something::Or::Other;
 my $config-file := "config.txt".IO.slurp;
 
+=head2 Assigning to C<Nil> produces a different value, usually C<Any>.
+
+Actually, assigning to C<Nil> L<reverts the variable to its default value|https://docs.perl6.org/type/Nil>. So:
+
+=begin code
+my @a = 4, 8, 15, 16;
+@a[2] = Nil;
+say @a; # OUTPUT: «[4 8 (Any) 16]␤»
+=end code
+
+In this case, C<Any> is the default value of an C<Array> element.
+
+You can purposefully assign C<Nil> as a default value:
+
+=begin code
+my %h is default(Nil) = a => Nil;
+dd %h; # OUTPUT: «Hash %h = {:a(Nil)}␤»
+=end code
+
+Or bind a value to C<Nil> if that is the result you want:
+
+=begin code
+@a[3] := Nil;
+say @a; # OUTPUT: «[4 8 (Any) Nil]␤»
+=end code
+
+This trap might be hidden in the result of functions, such as matches:
+
+=begin code
+my $result2 = 'abcdef' ~~ / dex /;
+say "Result2 is { $result2.^name }"; # OUTPUT: «Result2 is Any␤»
+=end code
+
+A L<C<Match> will be C<Nil>|https://docs.perl6.org/language/regexes#Literals> if it finds nothing; however it assigning C<Nil> to C<$result2> above will result in its default value, which is C<Any> as shown.
+
 =head1 Blocks
 
 =head2 Beware of empty "blocks"