always return a subroutine reference, regardless of whether or not the
attribute is read- or write-only.
+=item B<< $attr->insertion_order >>
+
+If this attribute has been inserted into a class, this returns a zero
+based index regarding the order of insertion.
+
=back
=head2 Informational predicates
=item B<< $attr->has_builder >>
+=item B<< $attr->has_insertion_order >>
+
+This will be I<false> if this attribute has not be inserted into a class
+
=back
=head2 Value management