A top-down pass chooses one parsimonious state assignment.
highlighted = computed this step
Choose a root state first
The root Fitch set contains only A, so the top-down pass assigns A to the root. A child keeps the parent's state when that state is available in the child's set.
parent state in child set⇒keep it
Assigned changes match the score
Count an edge as changed when parent and child assigned states differ. The induced mutation count is 2, matching the score 2.