MzScheme returns the unique value #<void> for expressions that
have undefined results in 
.
The procedure void takes any number of arguments and returns
#<void>. (void? v) returns #t if and only
if v is #<void>.
Non-global variables that are accessible but do not yet have a value are bound to the unique value #<undefined>. Such variables are created by letrec and letrec* expressions (see section 2.11), partially-initialized objects (see section 2.13), and partially-invoked units (see section 2.16).