Ruby/Reflection/module eval

class String
  module_eval %{def last(n)
                  self[-n, n]
                end}
end
p "Here"s a string.".last(7)               # => "string."
String.module_eval %{def last(n)
                       self[-n, n]
                     end}
p "Here"s a string.".last(7)               # => "string."

class Numeric
  [["add", "+"], ["subtract", "-"],
   ["multiply", "*",], ["divide", "/"]].each do |method, operator|
    module_eval %{ def #{method}_2
                     self #{operator} 2
                   end }
  end
end
4.add_2                                                  # => 6
10.divide_2                                              # => 5