evaluates its contents in-place.
  Can be named or anonymous,
  the named version is intended for inheritance
  layouts where any given section can be
  surrounded by the <%block> tag in order for
  it to become overrideable by inheriting
  templates, without the need to specify a
  top-level <%def> plus explicit call.
  Modified scoping and argument rules as well as a
  more strictly enforced usage scheme make it ideal
  for this purpose without at all replacing most
  other things that defs are still good for.
  Lots of new docs. [ticket:164]