Log4r is a comprehensive and flexible logging library written in Ruby
for use in Ruby programs.  It features a hierarchical logging system of
any number of levels, custom level names, logger inheritance, multiple
output destinations, execution tracing, custom formatting, thread
safteyness, XML and YAML configuration, and more.

Log4r is an adherent to the philosophy of logging using simple print
statements.  What Log4r adds to this philosophy is a flexible way of
controling the information being logged.  Log information can be sent
to any kind of destination and with varying degrees of importance.
Log4r is designed so that logging statements can remain in production
code with almost no extra computational cost.