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=item Bio::Das::Stylesheet

This class contains information about the stylesheet for a DAS source.

=back

=head2 PARALLEL AND SERIAL APIs

Bio::Das supports two distinct APIs. One is a parallel API which
allows you to make Das requests on two or more servers simultaneously.
This is highly efficient, but the API is slightly more difficult to
use.  The other is a serial API which supports only a single request
on a single service.  It is recommended for simple scripts or for
those where performance is not at a premium.

The two APIs use the same objects. You select which API to use when
you create the Das object with Bio::Das->new().

=head2 OBJECT CREATION

The public Bio::Das constructor is new().  It is used both for the
parallel and serial APIs.

B<Serial API object construction:>

=over 4

=item $das = Bio::Das->new(-server => $url, -dsn => $dsn, -aggregators=>\@aggregators);

Clients that will be accessing a single server exclusively can
indicate that they wish to use the serial APi by passing the
B<-server> argument.  The argument for B<-server> is the base name of
the DAS server (e.g. http://www.wormbase.org/db/das).  You may also
select the data source to use (e.g. "elegans") by passing the B<-dsn>
argument. B<-aggregators> is a list of aggregators as described
earlier.

The optional B<-proxy> argument will initialize the Bio::Das object
with an HTTP or HTTPS proxy (see also the proxy() method below).

=item $das = Bio::Das->new('http://das.server/cgi-bin/das',$dsn,$aggregators)

Shortcut for the above.

=back

B<Parallel API object construction:>

=over 4

=item $das = Bio::Das->new(-timeout       => $timeout,
                           -auth_callback => $authentication_callback,
                           -aggregators   => \@aggregators)

Create a new Bio::Das object, with the indicated timeout and optional
callback for authentication.  The timeout will be used to decide when
a server is not responding and to return a "can't connect" error.  Its
value is in seconds, and can be fractional (most systems will provide
millisecond resolution).  The authentication callback will be invoked
if the remote server challenges Bio::Das for authentication credentials.

Aggregators are used to build multilevel hierarchies out of the raw
features in the DAS stream.  For a description of aggregators, see
L<Bio::DB::GFF>, which uses exactly the same aggregator system as
Bio::Das.

The optional B<-proxy> argument will initialize the Bio::Das object
with an HTTP or HTTPS proxy (see also the proxy() method below).

If successful, this method returns a Bio::Das object.

=item $das = Bio::Das->new($timeout [,$authentication_callback])

Shortcut for the above.

=back

=head2 ACCESSOR METHODS

Once created, the Bio::Das object provides the following accessor methods:

=over 4

=item $proxy = $das->proxy([$new_proxy])

Get or set the proxy to use for accessing indicated servers.  Only
HTTP and HTTPS proxies are supported at the current time.

=item $callback = $das->auth_callback([$new_callback])

Get or set the callback to use when authentication is required.  See
the section "Authentication" for more details.

=item $timeout = $das->timeout([$new_timeout])

Get or set the timeout for slow servers.

=item $error  = $das->error

Get a string that describes the last error the module encountered whie
using the serial API.  If you are using the parallel API, then use the
request object's error() method to retrieve the error message from the
corresponding request.

=item $debug  = $das->debug([$debug_flag])

Get or set a flag that will turn on verbose debugging messages.

=item $das->add_aggregator($aggregator)

Aggregators allow you to dynamically build up more multipart features
from the simple one-part that are returned by Das servers.  The
concept of aggregation was introduced in the L<Bio::DB::GFF> module,
and is completely compatible with the Bio::Das implementation.  See
L<Bio::DB::GFF> and L<Bio::DB::GFF::Aggregator> for information on how
to create and use aggregators.

The add_aggregator() method will append an aggregator to the end of
the list of registered aggregators.  Three different argument types
are accepted: