Core Utilities for the 'rtemis' Ecosystem // 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).