is_existing is a vectorized wrapper to exists,
providing more information on failure (and with a simplified interface).
The assert_* functions return nothing but throw an error if
is_existing returns FALSE.
Arguments
x
Input to check.
envir
Passed to exists.
inherits
Passed to exists.
severity
How severe should the consequences of the assertion be?
Either "stop", "warning", "message", or "none".