with minor editorization by me.

Hstore improvements
* add operation hstore ? text - excat equivalent of exist()
* remove undocumented behaviour of contains operation with NULL value
* now 'key'::text=>NULL returns '"key"=>NULL' instead of NULL
* Add GIN support for contains and exist operations
* Add GiST support for exist operatiion
* improve regression tests

'exist' and 'defined' accordingly. Old names
are saved not mentioned in docs - for compatibility
with old applications.
Per discussion
http://archives.postgresql.org/pgsql-hackers/2006-10/msg00571.php

Per discussion
http://archives.postgresql.org/pgsql-hackers/2006-08/msg01409.php