Utilities used across packages of the rtemis ecosystem. Includes the msg() messaging system and the fmt() formatting system. Provides a library of S7 properties, test_* functions that return logical values, check_* functions that throw informative errors, and clean_* functions that return validated and coerced values. This code began as part of the rtemis package (<doi:10.32614/CRAN.package.rtemis>).