Fix up some code after aplanas' continued review

lib/abstract_walker.py

@@ -1,6 +1,10 @@
-class AbstractWalker:
+from abc import ABC, abstractmethod
+
+
+class AbstractWalker(ABC):
     """Just a duck type, most likely not needed by python, but I
     find interface classes preferable (java school)"""
 
+    @abstractmethod
     def call(self, node, is_source):
         pass