module documentation

Tests for traces.visitor.

Class BaseVisitorTest Tests for visitor.BaseVisitor.
Class custom_ast Tests a custom ast module.
Class _EnterVisitor Tests enter() by recording names.
Class _GenericVisitVisitor Tests generic_visit().
Class _LeaveVisitor Tests leave() by discarding names recorded by enter().
Class _VisitOrderVisitor Tests visit order.
Class _VisitReplaceVisitor Tests visit()'s node replacement functionality.