Function add

ui/src/core.ts:861–864

(k: S, card: Card)

      cards: Dict<Card> = {},
      changedB = box<B>(),
      add = (k: S, card: Card) => {
        cards[k] = card
        dirty = true
      },
      get = (k: S): Card | undefined => cards[k],
      items = (): Card[] => valuesOf(cards),
      drop = (k: S) => delete cards[k],