A value of type Arg.t will be randomly generated and passed to
the law function below.
Description of the test. This value is simply stored in the
result of the Check functor below for easy access by a test
driver.
The function that implements the law. The function should
return
true when the law holds for the input and
false if
the law does not hold. It may also raise the exception
Reins.Quickcheck.Trivial if the law only trivially applies to the input, in
which case a new input will be attempted.