'Add host or group to group'

def add_child(self, group, child):

    if (group in self.groups):
        g = self.groups[group]
        if (child in self.groups):
            g.add_child_group(self.groups[child])
        elif (child in self.hosts):
            g.add_host(self.hosts[child])
        else:
            raise AnsibleError(('%s    is    not    a    known    host    nor    group' % child))
        self._groups_dict_cache = {}
        display.debug(('Group    %s    now    contains    %s' % (group, child)))
    else:
        raise AnsibleError(('%s    is    not    a    known    group' % group))
