One level of the chart around a container: its children, the transitions
between them (merged per pair of children, so the view never shows more
than one arrow between two boxes), and "doors" for transitions crossing
the boundary. Linear in the size of the chart.
One level of the chart around a container: its children, the transitions between them (merged per pair of children, so the view never shows more than one arrow between two boxes), and "doors" for transitions crossing the boundary. Linear in the size of the chart.