'Map each of the aliases in the group to its ultimate destination.
@type aliasmap: L{dict} mapping L{bytes} to L{AliasBase}
@param aliasmap: A mapping of username to alias or group of aliases.
@type memo: L{None} or L{dict} of L{AliasBase}
@param memo: A record of the aliases already considered in the
resolution process.  If provided, C{memo} is modified to include
this alias.
@rtype: L{MultiWrapper}
@return: A message receiver which passes the message on to message
receivers for the ultimate destination of each alias in the group.'

def resolve(self, aliasmap, memo=None):

    if (memo is None):
        memo = {}
    r = []
    for a in self.aliases:
        r.append(a.resolve(aliasmap, memo))
    return MultiWrapper(filter(None, r))
