Adds monitor functionality to an arbitrary object by mixing the module with include. For example:

   require 'monitor.rb'

   buf = []
   buf.extend(MonitorMixin)
   empty_cond = buf.new_cond

   # consumer
   Thread.start do
     loop do
       buf.synchronize do
         empty_cond.wait_while { buf.empty? }
         print buf.shift
       end
     end
   end

   # producer
   while line = ARGF.gets
     buf.synchronize do
       buf.push(line)
       empty_cond.signal
     end
   end

The consumer thread waits for the producer thread to push a line to buf while buf.empty?, and the producer thread (main thread) reads a line from ARGF and push it to buf, then call empty_cond.signal.