July 15th Wednesday (七月　十五日　水曜日)

Weak Boxes

A weak box is similar to a normal box (A box is like a single-element vector, normally used as minimal mutable storage.), but when
the garbage collector can prove that the content value of a weak box is only reachable via weak references, the content of the weak box
is replaced with #f. A weak reference is a reference through a weak box, through a key reference in a weak hash table, through a value
in an ephemeron where the value can be replaced by #f, or through a custodian.