### A simple protocol where p sends a message to q, carrying a payload
### that is a subtype of what q expects.
###
### Copyright 2018 Alceste Scalas <alceste.scalas @ imperial.ac.uk>
### Released under the MIT License: https://opensource.org/licenses/MIT

s[p]: q⊕m(μ(t)r&m1.t) . end,

s[q]: p&m(μ(t)r&{m1.t,m2.t}) . end