rv creates a random vector of the specified length. The elements of
the vector are all equal to NA.
is.rv returns TRUE if its argument is a rv object, FALSE otherwise.
as.rv attempts to coerce its argument to the random vector (rv) type.
is.random returns TRUE or FALSE for each component of
the argument vector, depending on whether the component is a random variable
object.
is.rvobj tests whether its argument object is either of class
rv or of class rvsummary.
as.rvobj coerces its argument object to rv unless the object
is an rv object (is.rvobj(x) is TRUE).