Bug?? -- Alter table drop constraint doesn't seem to work on a primary key constraint in 7.2beta2

I am trying to use the new 'alter table drop constraint' syntax to drop
a primary key constraint (or foreign key constraint) on the 7.2beta2
code. It gives me the following error:

files31=# select version();
version
-------------------------------------------------------------
PostgreSQL 7.2b2 on i686-pc-linux-gnu, compiled by GCC 2.96
(1 row)

files31=# alter table XYF_FILES DROP CONSTRAINT XYF_FILES_PK;
ERROR: parser: parse error at or near ";"

When I look at the definition on the table through psql \d I can see
that this constraint does indeed exist. What am I doing wrong?

I am assuming that this syntax should work because it is documented in
the 7.2 docs.

thanks,
--Barry

PS I also notice that 'alter table drop constraint' doesn't appear to be
tested at all in the alter_table regression test. 'alter table add
constraint' is tested however.