Denotation of syntax and metaprogramming in contextual modal type theory   (CMTT)
  The modal logic S4 can be used via a Curry-Howard style correspondence to obtain a lambda-calculus. Modal (boxed) types are intuitively interpreted as `closed syntax of the calculus'. This lambda-calculus is called modal type theory --- this is the basic case of a more general contextual modal type theory, or CMTT.   CMTT has never been given a denotational semantics in which modal types are given denotation as closed syntax. We show how this can indeed be done, with a twist. We also use the denotation to prove some properties of the system. 
Murdoch Gabbay and Aleksandar Nanevski
cs.LO math.LO