def account_exists(self, account_name):
        """Check that L{account_name} is available in Cerebrum/Virthome. Names
        are case sensitive, but there should not exist two accounts with same
        name in lowercase, due to LDAP, so we have to check this too.

        (The combination if this call and account.write_db() is in no way
        atomic, but checking for availability lets us produce more meaningful
        error messages in (hopefully) most cases).

        @type account_name: str
        @param account_name: The account name to check

        @rtype: bool
        @return: True if account_name is available for use, False otherwise.
        """
        # Does not matter which one.
        ac = self.account_class(self.db)
        return not ac.uname_is_available(account_name)