While studying a bit of petri nets I encountered on Monotonicity Lemma, which says:
Let M and L be two markings of a net.
If M->M' for a finite sequence sigma, then (M+L)->(M'+L) for every marking L.
If M->for an infinite sequence sigma, then (M+L)-> for every marking L.
At the top of the arrows is sigma.
Does anyone understand what M+L mean in terms of markings? Should I add those markings together or is it a path where I add L to M?