class Sprockets::Bundle

Also see DirectiveProcessor.
:stubbed - Set of asset URIs to substract from the required set.
:required - Ordered Set of asset URIs to prepend
Uses pipeline metadata:
‘:required` URIs to the contents.
Internal: Bundle processor takes a single file asset and prepends all the

def self.call(input) 

def self.call(input)
  env  = input[:environment]
  type = input[:content_type]
  processed_uri = env.build_asset_uri(input[:filename], type: type, skip_bundle: true)
  find_required = proc { |uri| env.load(uri).metadata[:required] }
  required = Utils.dfs(processed_uri, &find_required)
  stubbed  = Utils.dfs(env.load(processed_uri).metadata[:stubbed], &find_required)
  required.subtract(stubbed)
  assets = required.map { |uri| env.load(uri) }
  dependencies = Set.new(input[:metadata][:dependencies])
  (required + stubbed).each do |uri|
    dependencies.merge(env.load(uri).metadata[:dependencies])
  end
  env.process_bundle_reducers(assets, env.load_bundle_reducers(type)).merge(dependencies: dependencies, included: assets.map(&:uri))
end