it won't work, security error both with module and class, what's wrong?

shikashi-sandbox.rb

#!/usr/bin/env ruby

require "rubygems"
require "shikashi"

include Shikashi

s = Sandbox.new
priv = Privileges.new

# allow execution of foo, print, puts in this object
priv.object(self).allow :foo, :print, :puts, :new

#inside the sandbox, only can use method foo on main and method times on instances of Fixnum
code = <<-EOS
  # class MP
  class MP
    def calc(a,b)
      a + b
    end

    def test(a)
      c = calc(10,20)
      puts "inside test "
      if (a)
        system('ls -l') # denied
      end
    end
  end

  #include AtlasMP
  #test
  AtlasMP.new.test

EOS

s.run(code, priv)