Clearly, there's something about singleton_method_added I don't
understand.  How can I set up callbacks for t4 and t5?


##### test code #####

#!/usr/bin/env ruby

class String
  def self.singleton_method_added(id)
    smn = id.id2name
    puts 'Adding singleton method '     + smn + " in class String."
  end

  def self.method_added(id)
    imn = id.id2name
    puts 'Adding instance method '      + imn + " in class String."
  end
end

a = 'aaa'

class String
  def self.t2c; puts '> t2c in String'; end           # <- callback
  def      t2i; puts '> t2i in String'; end           # <- callback
end
String.t2c
a.t2i

def String.t3; puts '> String.t3'; end                # <- callback
String.t3

def a.t4; puts '> a.t4'; end                          # <- no callback
a.t4

class String
  b = 'bbb'
  def b.t5; puts '> b.t5 in String'; end              # <- no callback
  b.t5
end



##### output #####

Adding singleton method singleton_method_added in class String.
Adding singleton method method_added in class String.
Adding singleton method t2c in class String.
Adding instance method t2i in class String.
> t2c in String
> t2i in String
Adding singleton method t3 in class String.
> String.t3
> a.t4
> b.t5 in String