on and after

Behavior attaches to its owner as an on or after clause inside the owner's create block (or inside a trait). The two words divide one moment: on runs before the action commits and may stop it; after runs once it has happened and reacts.

create the hive box
  aka hive, box
  scenery, openable
  in the Apiary
  states: humming, quiet

  A white wooden hive box, lid held down by a brick.

  on opening it
    refuse when it is humming: bees-disturbed
    change it to quiet
  end on

  after opening it
    phrase honey-smell
  end after

The division is enforced: refuse, refuse when, and must are parse errors inside an after body (parse.react-refusal). If a reaction wants to stop something, it is not a reaction; move it to on. Every clause closes with an end line matching its opener.