'Retrieve headers of one or more messages
This command is allowed in the Selected state.
@type messages: C{MessageSet} or L{str}
@param messages: A message sequence set
@type uid: C{bool}
@param uid: Indicates whether the message sequence set is of message
numbers or of unique message IDs.
@rtype: C{Deferred}
@return: A deferred whose callback is invoked with a dict mapping
message numbers to dicts of message headers, or whose errback is
invoked if there is an error.'

def fetchHeaders(self, messages, uid=0):

    return self._fetch(messages, useUID=uid, rfc822header=1)
