Example: ** Ostrich - Updating backdrop conditions manually.

    "Ostrich"

    Include Conditional Backdrops by John Clemens.

    The garden is a room. The path is east of the garden.

    The player carries the paper bag. The paper bag is wearable.

    The sky is a conditional backdrop.
    The grass is a conditional backdrop.

    Backdrop condition when the player wears the paper bag: it is absent.

    Backdrop condition for the sky: it is present.
    Backdrop condition for the grass: it is present.

    After wearing the paper bag:
        consider the move floating objects rule;
        say "You hide your head in the paper bag."

    After taking off the paper bag:
        consider the move floating objects rule;
        say "You bravely withdraw your head from the bag."

    Test me with "x sky / x grass / wear bag / x sky / x grass / remove bag / x sky / x grass".