Py3k-compatible syntax.
Ignore derived.
Py3k syntax straddling.
Py3k:  basestring gone, use string_types.
Py3k:  has_key delenda est.
Pyk3:  map() now returns a generator.
Pyk3:  filter() now returns a generator.
Py3k:  lists don't have a sort method.
Py3k:  ensure passing bytes to md5 hash.
Py3k:  LFSets are not sortable.
Py3k: Return an alwoys-greater object for sorting docids w/ missing values.
Py3k:  emulate xrange.
Py3k:  use 'reversed()'.
Py3k: use io.StringIO.
Pyk3:  compatible sorting using 'sorted()'.
Pyk3:  compatible use of 'long'.
Pyk3:  compatible use of 'iteritems'.
Pyk3:  compatible use of 'sys.maxint/maxsize'.
Py3k: reify result of range() for comparison w/ list.
Py3k: Deal with unary operator nodes.