barrel: prevent entrance of arbitrary objects

You could put e.g. tools into the barrel using the interaction menu. This would lead to a script error later, when the barrel did Contents()->GetLiquidType().
PS: And the RejectStack function is part of a private internal interface. I made that clear so the next one does not have to look it up again.

planet/Objects.ocd/Items.ocd/Tools.ocd/Barrel.ocd/Script.c

@@ -43,7 +43,7 @@ func CollectFromStack(object item)
 	}
 }
 
-func RejectStack(object item)
+private func RejectStack(object item)
 {
 	// Callback from stackable object: When should a stack entrance be rejected, if the object was not merged into the existing stacks?
 	if (Contents())
@@ -63,6 +63,13 @@ func RejectStack(object item)
 	}
 }
 
+public func RejectCollect(id def, object new_contents)
+{
+	// The barrel can only contain liquids.
+	if (RejectStack(new_contents)) return true;
+	return _inherited(def, new_contents, ...);
+}
+
 private func Hit()
 {
 	this->PlayBarrelHitSound();