R4RS 6.5.5  (number? object)   ==>  boolean
            (integer? object)  ==>  boolean

The numerical type predicates can be applied to any kind of argument,
including non-numbers. They return #T if the object is of the named
type, and otherwise they return #F.

(number? 42)   ==>  #t
(number? #\1)  ==>  #f
(integer? -1)  ==>  #t
