varname, if x has no natural orderAlso warns, if R does not have a comparison operator for x.
util_warn_unordered(x, varname)invisible(NULL)
Other robustness_functions:
util_as_valid_missing_codes(),
util_check_one_unique_value(),
util_correct_variable_use(),
util_empty(),
util_ensure_character(),
util_ensure_in(),
util_ensure_suggested(),
util_expect_scalar(),
util_fix_rstudio_bugs(),
util_is_integer(),
util_is_numeric_in(),
util_is_valid_missing_codes(),
util_match_arg(),
util_observations_in_subgroups(),
util_only_NAs(),
util_stop_if_not()