class Concurrent::Agent

def ns_initialize(initial, opts)
  @error_mode    = opts[:error_mode]
  @error_handler = opts[:error_handler]
  if @error_mode && !ERROR_MODES.include?(@error_mode)
    raise ArgumentError.new('unrecognized error mode')
  elsif @error_mode.nil?
    @error_mode = @error_handler ? :continue : :fail
  end
  @error_handler ||= DEFAULT_ERROR_HANDLER
  @validator     = opts.fetch(:validator, DEFAULT_VALIDATOR)
  @current       = Concurrent::AtomicReference.new(initial)
  @error         = Concurrent::AtomicReference.new(nil)
  @caller        = Concurrent::ThreadLocalVar.new(nil)
  @queue         = []
  self.observers = Collection::CopyOnNotifyObserverSet.new
end