box::register_S3_method makes an S3 method for a given generic and
class known inside a module.
box::register_S3_method(name, class, method)the name of the generic as a character string.
the class name.
the method to register.
register_S3_method is called for its side effect.
Methods for generics defined in the same module do not need to be
registered explicitly, and indeed should not be registered. However,
if the user wants to add a method for a known generic (defined outside the
module, e.g. print), then this needs to be made known
explicitly.
See the vignette at vignette('box', 'box') for more information about
defining S3 methods inside modules.