ty of holding on to the spec children. Some layout specs, like ASInsetLayoutSpec,
only require a single child.</p>

<p>For layout specs that require a known number of children (ASBackgroundLayoutSpec, for example)
a subclass can use the setChild method to set the &ldquo;primary&rdquo; child. It should then use this method
to set any other required children. Ideally a subclass would hide this from the user, and use the
setChild:forIndex: internally. For example, ASBackgroundLayoutSpec exposes a backgroundChi