Introduce a Targetable trait; Subs can attack bridges; May introduce subtle bugs due to previous stupid assumption that !selectable == !attackable.

OpenRA.Game/Traits/Building.cs

@@ -22,7 +22,6 @@ namespace OpenRA.Traits
 	{
 		public readonly int HP = 0;
 		public readonly ArmorType Armor = ArmorType.none;
-		public readonly string[] TargetType = {"Ground"};
 	}
 
 	public class BuildingInfo : OwnedActorInfo, ITraitInfo

OpenRA.Game/Traits/Targetable.cs

@@ -0,0 +1,26 @@
+#region Copyright & License Information
+/*
+ * Copyright 2007-2010 The OpenRA Developers (see AUTHORS)
+ * This file is part of OpenRA, which is free software. It is made 
+ * available to you under the terms of the GNU General Public License
+ * as published by the Free Software Foundation. For more information,
+ * see LICENSE.
+ */
+#endregion
+
+using System.Drawing;
+using OpenRA.Graphics;
+using System.Linq;
+
+namespace OpenRA.Traits
+{
+	public class TargetableInfo : TraitInfo<Targetable>
+	{
+		public readonly string[] TargetTypes = {};
+	}
+
+	public class Targetable
+	{
+		
+	}
+}