NewDAG news an empty DAG.

ErrPrematureStop is used to stop the Transformer chain for some purpose.

Plan defines the final actions should be executed.

PlanBuilder builds a Plan by applying a group of Transformer to an empty DAG.

TransformContext is used by Transformer.Transform.

Transformer transforms a DAG to a new version.

TransformerChain chains a group Transformer together.

WalkFunc defines the action should be taken when we walk through the DAG.