check is a generic function that tests the validity of a given
object.
check(object, …)an object to be tested for validity.
further arguments to be passed to the particular dispatched function.
returns TRUE if object passes the validity test for the
specific class and FALSE otherwise.
check tests if a given object meets the formal requirements of
being a valid object of its class. If the test fails, additional
warnings should be provided, giving some information why the test failed.