Allow passing multiple, or no, last locations

src/syai/game.py

+from typing import Iterable, Optional, Sequence, Union
+
 from pydot import graph_from_dot_file
 
 
@@ -40,8 +42,16 @@ class Board:
             self.ticket_graphs.setdefault(ticket, {}).setdefault(destination, set()).add(departure)
             self.ticket_graphs.setdefault("*", {}).setdefault(destination, set()).add(departure)
 
-    def get_possible_nodes(self, last: str, tickets: list[str]) -> list[str]:
-        possible = set((last,))
+    def get_possible_nodes(
+        self, tickets: Sequence[str], last: Optional[Union[Iterable[str], str]] = None
+    ) -> set[str]:
+        if last is None:
+            possible = set(self.ticket_graphs.get("*", {}).keys())
+        elif isinstance(last, str):
+            possible = set((last,))
+        else:
+            possible = set(last)
+
         for ticket in tickets:
             new_possible = set()
             for node in possible: