Available since 1.0 (source)
    *print-level* controls how many levels deep the printer will
    print nested objects. If it is bound to logical false, there is no
    limit. Otherwise, it must be bound to an integer indicating the maximum
    level to print. Each argument to print is at level 0; if an argument is a
    collection, its items are at level 1; and so on. If an object is a
    collection and is at a level greater than or equal to the value bound to
    *print-level*, the printer prints '#' to represent it. The root binding
    is nil indicating no limit.