#---
# Excerpted from "The ThoughtWorks Anthology",
# published by The Pragmatic Bookshelf.
# Copyrights apply to this code. It may not be used to create training material, 
# courses, books, articles, and the like. Contact us if you are in doubt.
# We make no guarantees that this code is fit for any purpose. 
# Visit http://www.pragmaticprogrammer.com/titles/twa for more book information.
#---
# Using closures with instance_eval


  item :secure_air_vent

  item(:acid_bath) do
    uses(acid) do
      type :hcl
      grade 5
    end
    uses(electricity(12))
  end

  item(:camera) do
    uses(electricity(1))
  end

  item(:small_power_plant) do
    provides(electricity(11))
    depends_on(:secure_air_vent)
  end