The schematic graph below represents the nullclines (solid curves) and a few selected trajectories (dotted curves) for a system composed of two proteins (X and Y) with a complex regulatory behavior. According to the graph, indicate whether each of the following statements is (Y) or is not (N) acceptable. Your answer would be a four-letter string composed of letters Y and N only, e.g. YYYY.

( ) The system is bistable.
( ) There are three steady states in the system.
( ) The system contains positive feedback.
( ) The system contains delayed negative feedback.