Two small lemmas
These are two lemmas that were needed when digging up the link between Prosa and Network Calculus (c.f., https://gitlab.mpi-sws.org/RT-PROOFS/project-administration/-/issues/23) and are currently in my prosa_missing
directory https://gitlab.mpi-sws.org/proux/nc-coq/-/tree/prosa-link/prosa_missing . I don't know if we want to integrate them into Prosa.