Nominal Unification of Higher Order Expressions with Recursive Let
  A sound and complete algorithm for nominal unification of higher-order expressions with a recursive let is described, and shown to run in non-deterministic polynomial time. We also explore specializations like nominal letrec-matching for plain expressions and for DAGs and determine the complexity of corresponding unification problems. 
Manfred Schmidt-Schau{\ss} and Temur Kutsia and Jordi Levy and Mateu   Villaret
cs.PL cs.LO