R4RS 6.5.5  (zero? number)      ==>  boolean
            (positive? number)  ==>  boolean
            (negative? number)  ==>  boolean
            (odd? integer)      ==>  boolean
            (even? integer)     ==>  boolean

These numerical predicates test a number for a particular property,
returning #T or #F.

See also: note 2 in description of =.
