To check the class simply pass a vector of the desired class.
To check that x does not include missing values pass
a single non-missing value (of the correct class).
To allow it to include missing values include a missing value.
To check that it only includes missing values only pass
a missing value (of the correct class).
To check the range of the values in x pass two non-missing values
(as well as the missing value if required).
To check that x only includes specific values
pass three or more non-missing values.
In the case of a factor ensure values has two levels to
check that the levels of x are an ordered superset of the levels of value
and three or more levels to check that they are identical.