"Return a sequence of less complex value than the one given.
In case the value is not further shrinkable, return `nil`."))
+;; There are two very basic types of values.
+;; * `SimpleValue` carries a value which is not shrinkable.
+;; * `ShrinkableValue` carries a `DomainValue` and function which
+;; specifies how the value should be shrunken.
+(deftype SimpleValue [v]
+;; These two types are used for the very basic generators. However
+;; it is perfectly possibly to define custom domain value types.
+(deftype ShrinkableValue [v f]
+ (value [this] (value v))
(arbitrary [this size] (DomainValue. this (this size)))