OrderedSlots
Every child fills a region of the host, and the host holds any number of them in the order they are composed - a JTabbedPane, which holds one page per tab and takes a new one at the position its child is composed at.
Two declarations naming the same region are equal, so a host that states its placement afresh on every recomposition states the same one.
Properties
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the call that fills one of this host's regions, written exactly as a caller of the container writes it - "SwingModifier.tab(title)" for a tab a modifier declares, "InternalFrame(...)" for a region a composable of the container's own scope fills. It is the text an error about this host's regions prints, and a caller acts on that text by typing it.