An Introduction to the Clocked Lambda Calculus
  We give a brief introduction to the clocked lambda calculus, an extension of the classical lambda calculus with a unary symbol tau used to witness the beta-steps. In contrast to the classical lambda calculus, this extension is infinitary strongly normalising and infinitary confluent. The infinitary normal forms are enriched Boehm Trees, which we call clocked Boehm Trees. 
J\"org Endrullis, Dimitri Hendriks, Jan Willem Klop, Andrew Polonsky
cs.LO