Exercise 2: A Nested "backend" Group — Possible Solution
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[backend:children]
webservers
dbservers

Explanation:

This mirrors the chapter's own [production:children] example exactly,
just renamed to "backend" and listing webservers and dbservers as its
child groups, one per line, rather than typing out every individual
host directly under [backend].

-- What hosts would a play with hosts: backend actually target? --
--
-- Every host that belongs to EITHER child group -- meaning, using the
-- chapter's own earlier example inventory, that's web1.example.com
-- and web2.example.com (both members of webservers) PLUS
-- db1.example.com (a member of dbservers), combined into one single
-- target set. A play targeting hosts: backend runs against all three
-- of these hosts in one go, exactly as if they'd all been listed
-- directly under one group -- the :children syntax just lets that
-- combined group be expressed by referencing the two existing groups
-- rather than duplicating every host name a second time.

WHY THIS WORKS AS AN ANSWER
------------------------------
This builds the nested group using the chapter's own :children syntax
correctly, then explicitly lists out which specific hosts would
actually be targeted by referencing the chapter's own earlier
webservers/dbservers example inventory, rather than answering only in
the abstract.