define trait

A trait is a reusable bundle of data, states, scores, phrases, and behavior that entities compose with. define trait <name> … end trait declares one. The block below carries one of everything:

define trait feedable
  data
    food: entity
    ration: optional number
    diet: one of hay, seed, fish
  states, reversible: hungry, content
  score fed worth 10

  phrases en-US
    no-food:
      You have nothing {the item} would want.
    already-fed:
      {capitalize the item} has had quite enough already.
    fed:
      The feed vanishes in seconds.

  on feeding it
    the player must have its food: no-food
    it must be hungry: already-fed
    change it to content
    award fed
    phrase fed
  end on
end trait

The parts, top to bottom:

• data declares fields. Each is <name>: <type>, where the type is entity, number, name, or one of <word>, <word>, … for a fixed set. optional before the type makes the field omittable; , starts <value> (not shown here) gives a default. Fields are read and written with its <field> / set its <field> to ….
• states and score are the same owner lines as everywhere else, here owned by the trait: every entity composed with feedable gets the hungry/content states and can award fed.
• phrases is a locale block scoped to the trait.
• on/after clauses are the trait's behavior, with it meaning the composed entity.

An entity picks the trait up on a composition line, passing its data with with:

create the pygmy goats
  aka goats
  feedable with food the handful of feed and diet hay
  in the Petting Zoo

A state name declared by two different composed traits on one entity is a collision (analysis.state-collision), caught at load time.