------------------------------------------------- Module#module_function
     module_function(symbol, ...)    => self
------------------------------------------------------------------------
     Creates module functions for the named methods. These functions may
     be called with the module as a receiver, and also become available
     as instance methods to classes that mix in the module. Module
     functions are copies of the original, and so may be changed
     independently. The instance-method versions are made private. If
     used with no arguments, subsequently defined methods become module
     functions.

        module Mod
          def one
            "This is one"
          end
          module_function :one
        end
        class Cls
          include Mod
          def callOne
            one
          end
        end
        Mod.one     #=> "This is one"
        c = Cls.new
        c.callOne   #=> "This is one"
        module Mod
          def one
            "This is the new one"
          end
        end
        Mod.one     #=> "This is one"
        c.callOne   #=> "This is the new one"