Returns the iden that starts with prefix.  Prints out error and returns None if it doesn't match
        exactly one.

async def _match_idens(self, core, prefix):
        '''
        Returns the iden that starts with prefix.  Prints out error and returns None if it doesn't match
        exactly one.
        '''
        idens = [iden for iden, trig in await core.listTriggers()]
        matches = [iden for iden in idens if iden.startswith(prefix)]
        if len(matches) == 1:
            return matches[0]
        elif len(matches) == 0:
            self.printf('Error: provided iden does not match any valid authorized triggers')
        else:
            self.printf('Error: provided iden matches more than one trigger')
        return None