Creates a class that is either #f or the supplied class. This function is useful in type expressions. It captures the common idiom of returning an instance of a particular type or the value #f. The expression
false-or(<integer>)
is equivalent to the expression
type-union(<integer>, singleton(#f))
| arg | An instance of <type>. The alternate type. |
| val | An instance of <type>. The resulting type-union. |