These functions check whether a value was specified as an argument to a
function.
Usage
check_missing(x)
assert_missing(x)
test_missing(x)
Value
Depending on the function prefix:
If the check is successful, assert_missing() returns x invisibly,
whereas check_missing() and test_missing() return TRUE.
If the check is not successful, assert_missing() throws an error message,
test_missing() returns FALSE, and check_missing() returns a string with
the error message.