These functions examine the expression of a quosure with a predicate.
quo_is_missing(quo) quo_is_symbol(quo) quo_is_lang(quo) quo_is_symbolic(quo) quo_is_null(quo)
When missing arguments are captured as quosures, either through
quos(), they are returned as an empty quosure. These
quosures contain the missing argument and typically
have the empty environment as enclosure.