Scoping and locating definitions

Consider the following:

lizzy:~% cat r
require 'lib'
# from 'lib' import 'Foo'

f = Foo.new("string")
p f.bar

p 'abc'.rot13
lizzy:~% cat lib.rb 
class String
  def rot13
    tr 'A-Za-z', 'N-ZA-Mn-za-m'
  end
end

class Foo
  def initialize(s)
    @s = s
  end

  def bar
    @s.rot13
  end
end
lizzy:~% ruby r
"fgevat"
"nop"
lizzy:~% 

What if I wanted String#rot13 to _not_ be visible in file `r' (because I
didn't ask for it)?  How do I contain the change to String to lib.rb only? Or
perhaps to Foo?

(The idea is to be able to reduce side-effects from require'ing files.  Python
forces one to be explicit about this, and that seems like a Good Thing.)

On a related note: how does one go about finding _where_ String#rot13 was
defined? One possible way would be for .methods to return an Array of
BoundMethods instead of Strings and have file and linenumber information be
part of BoundMethod (if possible).

Thoughts?

-- 
Jos Backus
jos at catnook.com