nulls in GIN index

http://www.postgresql.org/docs/9.1/static/gin-implementation.html
"""As of PostgreSQL 9.1, NULL key values can be included in the index. Also,
placeholder NULLs are included in the index for indexed items that are NULL
or contain no keys according to extractValue. This allows searches that
should find empty items to do so."""

How do I define an index that includes nulls then?



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