[ASDL cleanup] Remove unnecessary Obj type

Add comments.

asdl/pybase.py

@@ -1,5 +1,5 @@
 #!/usr/bin/env python2
-"""pybase.py."""
+"""asdl/pybase.py is a runtime library for ASDL in Python"""
 from __future__ import print_function
 
 from mycpp import mylib
@@ -10,21 +10,16 @@
     from asdl.runtime import TraversalState
 
 
-class Obj(object):
-    # NOTE: We're using CAPS for these static fields, since they are constant at
-    # runtime after metaprogramming.
-    ASDL_TYPE = None  # Used for type checking
-
-
 class SimpleObj(int):
-    """Base type of simple sum types."""
-    # TODO: Get rid of this indirection?  Although mycpp might use it.
+    """Base type of simple sum types, which are integers."""
+    # TODO: Get rid of this class?  mycpp uses it to tell if it should generate
+    # h8_id or h8_id*.
     pass
 
 
-class CompoundObj(Obj):
-    # The tag is set for variant types, which are subclasses of sum
-    # types.  Never set for product types.
+class CompoundObj(object):
+    # The tag is set for variant types, which are subclasses of sum types.
+    # It's not set for product types.
     _type_tag = 0  # Starts at 1.  Zero is invalid
 
     def PrettyTree(self, do_abbrev, trav=None):
@@ -33,6 +28,8 @@ def PrettyTree(self, do_abbrev, trav=None):
 
     def __repr__(self):
         # type: () -> str
+        """Print this ASDL object nicely."""
+
         # TODO: Break this circular dependency.
         from asdl import format as fmt