done getting player's projectiles to collide with enemies successfully, and tested (although the code of the enemy reacting to the player is not the best)

fcore/game-update.sml

@@ -7,15 +7,11 @@ struct
 
       val player = Player.runPhysicsAndInput (game, input)
 
-      (* check and react to enemy collisions with player projectile *)
-      val enemies =
-        ProjectileEnemy.checkCollisions
-          (#projectiles player, enemies, enemyTree)
-      val enemyTree = Enemy.generateTree enemies
-
       (* check player-enemy collisions and react *)
       val (player, enemies, enemyTree) =
-        PlayerEnemy.checkCollisions (player, enemies, enemyTree)
+        PlayerEnemy.checkCollisions
+          (player, enemies, enemyTree, #projectiles player)
+
     in
       { player = player
       , walls = walls