'Takes host_ids (a list of host ids) and a function which should
add or take one from an input. Modify the string representation
and the vuln_amount of the host_ids found in the model by adding or taking
one vulnerability from them, according to the plus_one_or_minus_one
function. Return None.'

def _modify_vuln_amounts_of_hosts_in_model(self, host_ids, plus_one_or_minus_one):

    relevant_host_ids = filter(self._is_host_in_model_by_host_id, host_ids)
    host_iters = map((lambda h: self.host_id_to_iter[h]), relevant_host_ids)
    vuln_amount_of_those_hosts = map(self._get_vuln_amount_from_model, host_iters)
    new_vuln_amounts = map(plus_one_or_minus_one, vuln_amount_of_those_hosts)
    map(self._modify_vuln_amount_of_single_host_in_model, relevant_host_ids, new_vuln_amounts)
