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]
  # TODO: Rebuilding this URI is a bit of a smell
  processed_uri = AssetURI.build(input[:filename], type: type, skip_bundle: true)
  cache = Hash.new do |h, uri|
    h[uri] = env.find_asset_by_uri(uri)
  end
  find_required = proc { |uri| cache[uri].metadata[:required] }
  required = Utils.dfs(processed_uri, &find_required)
  stubbed  = Utils.dfs(cache[processed_uri].metadata[:stubbed], &find_required)
  required.subtract(stubbed)
  assets = required.map { |uri| cache[uri] }
  env.process_bundle_reducers(assets, env.unwrap_bundle_reducers(type)).merge(included: assets.map(&:uri))
end