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Upset and downset of concept object now are iterable-returning methods instead
of properties (backwards incompatible).
Changed concept object minimal generating properties to be computed on request
instead of being precomputed during lattice build; changed minimal and
attributes from properties to methods (backwards incompatible).
Improved unicode support.
Changed pickling of lattice and concepts.
Changed ordering of ``downset`` and ``lower_neighbors`` to longlex.