Improved replacement system

core/src/mindustry/entities/bullet/BulletType.java

@@ -85,7 +85,7 @@ public abstract class BulletType extends Content{
     public float fragAngle = 0f;
     public int fragBullets = 9;
     public float fragVelocityMin = 0.2f, fragVelocityMax = 1f, fragLifeMin = 1f, fragLifeMax = 1f;
-    public BulletType fragBullet = null;
+    public @Nullable BulletType fragBullet = null;
     public Color hitColor = Color.white;
 
     public Color trailColor = Pal.missileYellowBack;

core/src/mindustry/entities/comp/BuildingComp.java

@@ -849,6 +849,13 @@ abstract class BuildingComp implements Posc, Teamc, Healthc, Buildingc, Timerc,
         }
     }
 
+    /**
+     * Called when a block is placed over some other blocks. This seq will always have at least one item.
+     * Should load some previous state, if necessary. */
+    public void overwrote(Seq<Building> previous){
+
+    }
+
     public void onRemoved(){
     }