     @_count = {}
     @_state = {}
     @_cycles = {}
-    @_unmet = 0
 
   # Subscribe to the `requests` and `responses` channels.
   listen: ->

  # zero, it is rescheduled.
  _progress: (keys) ->
    for key in keys
      @_unmet--
      unless --@_count[key]
        @_reschedule key


      when 'done' then return
      when undefined then @_reqs.push key
    (@deps[key] ?= []).push source
    @_count[source]++
  
  # Take the unmet dependencies from the latest request and push
