all: # keys must be unique, i.e. only one 'hosts' per group
    hosts:
        test1:
        test2:
            host_var: value
    vars:
        group_all_var: value
    children:   # key order does not matter, indentation does
        other_group:
            children:
                group_x:
                    hosts:
                        test5   # Note that one machine will work without a colon
                #group_x:
                #    hosts:
                #        test5  # But this won't
                #        test7  #
                group_y:
                    hosts:
                        test6:  # So always use a colon
            vars:
                g2_var2: value3
            hosts:
                test4:
                    ansible_host: 127.0.0.1
        last_group:
            hosts:
                test1 # same host as above, additional group membership
            vars:
                group_last_var: value