class OptionParser

# File lib/optparse_ex.rb, line 9
def on(attr, *args, &block)
  if attr.is_a?(Symbol)
    self.class.class_eval do
      unless method_defined?(attr)
        attr_accessor attr 
      else
        raise "Method #{attr.to_s} is already defined in OptionParser class"
      end
    end
    unless args.first =~ /^\-/
      self.send(attr.to_s+"=", args.shift)
    end
    _on(*args) do |i|
      self.send(attr.to_s+"=", i)
      block.call(i) if block
    end
  else
    args.unshift attr
    _on(*args, block)
  end
end