// Doors toggle between closed (solid) and open (passable).
pub fn toggle(ctx) {
    if ctx.is_solid() {
        ctx.set_solid(false);
        ctx.set_variant("open");
        ctx.message("You open the door.");
    } else {
        ctx.set_solid(true);
        ctx.set_variant("");
        ctx.message("You close the door.");
    }
}