Example:

foo.cfg:

  [foo]
  url = ...

bar.cfg:

  [buildout]
  extends = foo.cfg

  [foo]
  url =
  inline = blablah

Recipes must not distinguish the existence of an option from an empty
option value. It's a common mistake in recipes because that's natural
to do that in Python. However, buildout has no syntax to remove an
option when overriding a section.