Switch to itertools chaining from repeated updates in concatenate follow

Author: LordDarkula

interegular/fsm.py

@@ -350,15 +350,17 @@ def follow(current, new_transition):
                 next FSM if we reach the end of the current one
                 TODO: improve all follow() implementations to allow for dead metastates?
             """
-            next_states = set()
-            for (i, substate) in current:
-                fsm = fsms[i]
-                current_vertex: TransitionKey = new_to_old[i][new_transition]
-                if substate in fsm.map and current_vertex in fsm.map[substate]:
-                    next_states.update(connect_all(i, fsm.map[substate][current_vertex]))
+            def next_states():
+                for (i, substate) in current:
+                    fsm = fsms[i]
+                    current_vertex: TransitionKey = new_to_old[i][new_transition]
+                    if substate in fsm.map and current_vertex in fsm.map[substate]:
+                        yield connect_all(i, fsm.map[substate][current_vertex])
+
+            next_states_set = frozenset(chain.from_iterable(next_states()))
             if not next_states:
                 raise OblivionError
-            return frozenset(next_states)
+            return next_states_set
 
         return crawl(alphabet, initial, final, follow)