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  $db->delete("foo"); # for hashes
  $db->delete(1); # for arrays

=item * clear()
X<clear>

Deletes B<all> hash keys or array elements. Takes no arguments. No return
value.

  $db->clear(); # hashes or arrays

=item * lock() / unlock() / lock_exclusive() / lock_shared()
X<lock>
X<unlock>
X<lock_exclusive>
X<lock_shared>

q.v. L</LOCKING> for more info.

=item * optimize()
X<optimize>

This will compress the datafile so that it takes up as little space as possible.
There is a freespace manager so that when space is freed up, it is used before
extending the size of the datafile. But, that freespace just sits in the
datafile unless C<optimize()> is called.

C<optimize> basically copies everything into a new database, so, if it is
in version 1.0003 format, it will be upgraded.

=item * import()
X<import>

Unlike simple assignment, C<import()> does not tie the right-hand side. Instead,
a copy of your data is put into the DB. C<import()> takes either an arrayref (if
your DB is an array) or a hashref (if your DB is a hash). C<import()> will die
if anything else is passed in.

=item * export()
X<export>

This returns a complete copy of the data structure at the point you do the export.
This copy is in RAM, not on disk like the DB is.

=item * begin_work() / commit() / rollback()

These are the transactional functions. L</TRANSACTIONS> for more information.

=item * supports( $option )
X<supports>

This returns a boolean indicating whether this instance of DBM::Deep
supports that feature. C<$option> can be one of:

=over 4

=item * transactions
X<translation>

=item * unicode
X<unicode>

=back

=item * db_version()
X<db_version>

This returns the version of the database format that the current database
is in. This is specified as the earliest version of DBM::Deep that supports
it.

For the File back end, this will be 1.0003 or 2.

For the DBI back end, it is currently always 1.0020.

=back

=head2 Hashes

For hashes, DBM::Deep supports all the common methods described above, and the
following additional methods: C<first_key()> and C<next_key()>.

=over

=item * first_key()
X<first_key>

Returns the "first" key in the hash. As with built-in Perl hashes, keys are
fetched in an undefined order (which appears random). Takes no arguments,
returns the key as a scalar value.

  my $key = $db->first_key();

=item * next_key()
X<next_key>

Returns the "next" key in the hash, given the previous one as the sole argument.
Returns undef if there are no more keys to be fetched.

  $key = $db->next_key($key);

=back

Here are some examples of using hashes:

  my $db = DBM::Deep->new( "foo.db" );

  $db->put("foo", "bar");
  print "foo: " . $db->get("foo") . "\n";

  $db->put("baz", {}); # new child hash ref
  $db->get("baz")->put("buz", "biz");
  print "buz: " . $db->get("baz")->get("buz") . "\n";

  my $key = $db->first_key();
  while ($key) {
      print "$key: " . $db->get($key) . "\n";
      $key = $db->next_key($key);
  }

  if ($db->exists("foo")) { $db->delete("foo"); }