Get the frame of the given node.
Get the frame of the given node. If the node isn't in any frame, return None.
Create a frame.
Create a frame. If it exists, nothing happen
Mark a node enter into frame.
Mark a node enter into frame. The node cannot be in two frames at the same time
node
the name of the frame
Manage frame in scheduler. When scheduler execute nodes, it may enter a
frame
. Before scheduler leave a frame, it must make sure all nodes in that frames has been run.