Checks if .then and .else are fine. If they have supported classes and if their length is the same.
check_thenelse_OX(.then, .else)(list, atomic, NULL)
A positive-replacement.
NOTE, that if .then is not specified directly by named argument then the
first argument from ... will be taken.
(list, atomic, NULL)
A negative-replacement. Should be of length equal to length of .then, or
single value or NULL.
NOTE, that if .else is not specified directly by named argument then the
last argument from ... will be considered as a replacement.
NULL invisible.