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Creates a new instance of this class. The first layer of the stack can be pre-populated by passing 
a hash ref (I<$default_hash>) whos keys and values will be copied.

=back

=head2 INSTANCE METHODS

=over 4

=item exists ( $key )

Checks if data with the key I<$key> exists anywhere in the stack. Returns the layer (from the top) 
in which the key is first found. -1 for top-most layer, -2 for second top-most layer and so forth. 
Returns 0 if the key is not found at all.

=item delete ( $key )

Deletes the value of I<$key> at the top of the stack. If an entry with the same key exists down the 
stack C<get> and C<exists> will operate on that value.

=item get ( $key )

Returns the value of I<$key> or undef it the key doesn't exist. Will search down the stack 
to find the value.

=item set ( $key => $value )

=item put ( $key => $value ) 

Sets the value of I<$key> to I<$value>.

=item keys ()

Returns a list of the keys defined in all layers of the stack.

=item top_keys ()

Returns a list of the keys defined in just the top layer of the stack.

=item push ()

Creates a new current map in the stack.

=item pop ()

Removes the current stack entry and returns it. Will throw an error if called when there is only 
one map in the stack.

=item size ()

Returns the length of the stack, i.e. how many layers deep it currently is.

=back

=head1 SEE ALSO

L<Tie::Hash::Stack>

L<Data::StackedHash>

=head1 BUGS AND LIMITATIONS

Please report any bugs or feature requests to C<bug-data-stackedmap@rt.cpan.org>, 
or through the web interface at L<http://rt.cpan.org>.

=head1 AUTHOR

Claes Jakobsson C<< <claesjac@cpan.org> >>

=head1 LICENCE AND COPYRIGHT

Copyright (c) 2007, Claes Jakobsson C<< <claesjac@cpan.org> >>. All rights reserved.

This module is free software; you can redistribute it and/or
modify it under the same terms as Perl itself. See L<perlartistic>.

=head1 DISCLAIMER OF WARRANTY

BECAUSE THIS SOFTWARE IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY
FOR THE SOFTWARE, TO THE EXTENT PERMITTED BY APPLICABLE LAW. EXCEPT WHEN
OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES
PROVIDE THE SOFTWARE "AS IS" WITHOUT WARRANTY OF ANY KIND, EITHER
EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE
ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE SOFTWARE IS WITH
YOU. SHOULD THE SOFTWARE PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL
NECESSARY SERVICING, REPAIR, OR CORRECTION.

IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR
REDISTRIBUTE THE SOFTWARE AS PERMITTED BY THE ABOVE LICENCE, BE
LIABLE TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL,
OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE
THE SOFTWARE (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA BEING
RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A
FAILURE OF THE SOFTWARE TO OPERATE WITH ANY OTHER SOFTWARE), EVEN IF
SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF
SUCH DAMAGES.

=cut