How to workaround DROP CONSTRAINT [ IF EXISTS ] in Postgres version 8.1?

Hello,

I found that DROP CONSTRAINT [ IF EXISTS ] is available in Postgres 9.1
http://developer.postgresql.org/pgdocs/postgres/sql-altertable.html
http://developer.postgresql.org/pgdocs/postgres/sql-altertable.html

But it is not avaiable in Postgres 8.1
http://www.postgresql.org/docs/8.1/interactive/sql-altertable.html
http://www.postgresql.org/docs/8.1/interactive/sql-altertable.html

Is there a way to achieve this on Postgres 8.1?

Thanks a lot
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