fixed:

1. potions being walked over
2. spawning on stairs
3. enemies walking under players
4. merchants not being aggresive
5. potions losing effect when sending pass signal (cleared by player::start_turn())’

src/position.cc

@@ -67,7 +67,7 @@ std::vector<position> remove_from_list(const std::vector<position>
 
 void remove_from_list(std::vector<position>
                       &positions,
-                      position &excluded) {
+                      const position &excluded) {
         for (auto i = positions.begin(); i != positions.end(); ++i)
                 if (*i == excluded) {
                         positions.erase(i);