On Dart-Zobel Algorithm for Testing Regular Type Inclusion
  This paper answers open questions about the correctness and the completeness of Dart-Zobel algorithm for testing the inclusion relation between two regular types. We show that the algorithm is incorrect for regular types. We also prove that the algorithm is complete for regular types as well as correct for tuple distributive regular types. Also presented is a simplified version of Dart-Zobel algorithm for tuple distributive regular types. 
Lunjin Lu and John G. Cleary