Let custom schema node start out with empty children

sometimes that's just necessary

tailbone/forms2/core.py

@@ -145,10 +145,20 @@ class CustomSchemaNode(SQLAlchemySchemaNode):
             msg = 'excludes and includes are mutually exclusive.'
             raise ValueError(msg)
 
-        properties = sorted(self.inspector.attrs, key=_creation_order)
         # sorted to maintain the order in which the attributes
         # are defined
-        for name in includes or [item.key for item in properties]:
+        properties = sorted(self.inspector.attrs, key=_creation_order)
+        if excludes:
+            if includes:
+                raise ValueError("Must pass includes *or* excludes, but not both")
+            supported = [prop.key for prop in properties
+                         if prop.key not in excludes]
+        elif includes:
+            supported = includes
+        elif includes is not None:
+            supported = []
+
+        for name in supported:
             prop = self.inspector.attrs.get(name, name)
 
             if name in excludes or (includes and name not in includes):