  Concurrent Kleene Algebra (CKA) is a recently proposed algebraic structure by
Hoare and collaborators that unifies the laws of concurrent programming. The
unifying power of CKA rests largely on the so-called exchange law that
describes how concurrent and sequential composition operators can be
interchanged. Based on extensive theoretical work on true concurrency in the
past, this paper extends Gischer's pomset model with least fixed point
operators and formalizes the program refinement relation by \'{E}sik's
monotonic bijective morphisms to construct a partial order model of CKA. The
existence of such a model is relevant when we want to prove and disprove
properties about concurrent programs with loops. In particular, it gives a
foundation for the analysis of programs that concurrently access relaxed memory
as shown in subsequent work.
