Net::OAI::ListIdentifiers - Results of the ListIdentifiers OAI-PMH verb.
This class is based on
Net::OAI::Base and inherits the general methods expect for the following:
Returns the Net::OAI::Record::Header object for the next OAI record in the response,
undef if none remain. resumptionToken handling is performed automagically if the original request was listAllIdentifiers().