class Concurrent::Promises::AbstractEventFuture

def add_callback(method, *args)
  state = internal_state
  if state.resolved?
    call_callback method, state, args
  else
    @Callbacks.push [method, args]
    state = internal_state
    # take back if it was resolved in the meanwhile
    call_callbacks state if state.resolved?
  end
  self
end