gistfile1.rb

def control(opts=nil, &block)
  op = self
  @control = Proc.new do
    if opts and
        opts[:with_capability] and
        !driver.respond_to?(opts[:with_capability])
      raise Deltacloud::BackendFeatureUnsupported.new('501', nil,
                                                      "#{opts[:with_capability]} not supported by backend", nil)
    end
    op.validate(params)
    instance_eval(&block)
  end
end

#this:
  operation :destroy do
    description "Destroy given instance credential if backend supports this."
    param :id,  :string,  :required
    control do
      unless driver.respond_to?(:destroy_key)
        raise Deltacloud::BackendFeatureUnsupported.new('501',
          'Creating instance credentials is not supported in backend')
      end
      driver.destroy_key(credentials, { :key_name => params[:id]})
      redirect(keys_url)
    end
  end

#becomes this:
  operation :destroy do
    description "Destroy given instance credential if backend supports this."
    param :id,  :string,  :required
    control(:with_capability => :destroy_key) do
      driver.destroy_key(credentials, { :key_name => params[:id]})
      redirect(keys_url)
    end
  end