The Logging concept is borrowed from Python's Concepts.qll.

This encodes what may be present in log output from stdlib Logger instances, covering the following methods:

• add/log
• debug, error, fatal, info, unknown, and warn
• <<
• progname=

add/log only execute their block and include its return value in log output when the message argument is nil (this is also its default value). See the test cases or Logger#add for details.