action may never end if its arguments change while it is running

Imagine a simple action call `a(f())` where `f()` is some fluent. This is implicitly translated into `{ start(a(f())); end(a(f())); }`. Now if some exogenous event changes the value of `f()` before `end(a(f()))` becomes possible, `end(a(f()))` suddenly refers to a different action call which has never been started and therefore never ends :-(