Internal functions to be called from within another functions.
.check_classes(x, ...).check_factors(x, ...)
.check_labels(x, ...)
.check_length(...)
Error message if the test fails. Nothing if the test passes.
expected objects (character vector)
arguments to be checked.
.check_classes checks (tests) if the arguments passed to functions comply with their expected classes.
.check_labels checks (tests) if the character arguments passed to functions comply with their expected content.
.check_length checks (tests) if the length of objects match.