RPC::XML perl module, valid values for signature?

Greetings all,

Can anyone shed light on how signatures should be used in RPC::XML::Server? 

The documentation is fairly vague...

=====

http://search.cpan.org/~rjray/RPC-XML-0.74/lib/RPC/XML/Server.pm#Methods

signature

    A list reference of the signatures by which this routine may be invoked. Every method has at least one signature. Though less efficient for cases of exactly one signature, a list reference is always used for sake of consistency.

=====

What are the valid values for the signature field, and what do they represent? 

Below is an example where you can see the signature field in action... what you see are the fields include things like "array", "array string", "array array"...Is there a finite set of values for these fields, and if so, what are they? 

# This is an example program from Programming Web Services with Perl,
# by Randy Ray and Pavel Kulchenko

use strict;
use warnings;

use XRFortune;
use RPC::XML::Server;

# The object created by the constructor will be used to
# chain on a set of calls to add_proc() (which means it
# isn't necessary to deal with the local routines being
# called as methods), and then drop directly into the
# server_loop() method.

RPC::XML::Server->new(port => 31135)
    # add_proc can take a pre-created RPC::XML::Procedure
    # object, or a hash reference.
    # It gets called once for each routine being made
    # public by the server.
    ->add_proc({ name => 'fortune',
                 signature => [ 'array',
                                'array string',
                                'array array' ],
                 code => \&XRFortune::fortune })

 # This method will only return after a signal interrupts
    # it.
    ->server_loop;

exit;


Thanks!
Ted



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