suggestion for Joe Cancilla question https://class.coursera.org/proglang-2012-001/forum/thread?thread_id=3627&post_id=16486#post-16486

mysum.rb

class Object
  def mysum
    0
  end
end

class Fixnum  # The class belonging to small integers
  def mysum   
      self    # The sum of an integer is just itself
  end
end

class Array   # The class of for Arrays
  def mysum   # Sum up the result of calling mysum on all the elements
      inject(0) do |acc,elt|
        acc + elt.mysum
      end
  end
end

a = [1, [2, "2.5", 3], [4, "hello", [5, [6, "world", 7]]], 8, [9, 10], "42"]
puts a.mysum