nbt support WIP and proper container API

src/main/java/carpet/script/value/AbstractListValue.java

@@ -1,9 +1,15 @@
 package carpet.script.value;
 
+import carpet.script.exception.InternalExpressionException;
+
 import java.util.Iterator;
 
 public abstract class AbstractListValue extends Value
 {
     public abstract Iterator<Value> iterator();
     public void fatality() { }
+    public void append(Value v)
+    {
+        throw new InternalExpressionException("Cannot append a value to abstract list");
+    }
 }