class ERB

def def_module(methodname='erb')

end
include MyModule
class MyClass
MyModule = erb.def_module('render(arg1, arg2)')
erb.filename = filename
erb = ERB.new(File.read(filename))
filename = 'example.rhtml' # 'arg1' and 'arg2' are used in example.rhtml
example:

Create unnamed module, define _methodname_ as instance method of it, and return it. 
def def_module(methodname='erb')
  mod = Module.new
  def_method(mod, methodname, @filename || '(ERB)')
  mod
end