Past exam of the computer science course of the University of Oxford / 2015 / Concurrency / 1 / b / Solution

Take a fresh CSP event $h$ and defineThe apparent guard is removed by CSP hiding. Each unfolding can perform a CSP internal action and return to the same situation without any visible CSP event. Operationally this is an infinite internal loop, a CSP divergence, so the equation is not a guarded recursion. This is precisely why a syntactic test that counts every occurrence of $\to$ would be wrong.