Adding Foreign Key Constraint To Existing Table

I've examined the 9.0 manual page on alter table without seeing how to add
a foreign key constraint to a column.

I needed to make changes on a couple of existing tables which could be
accomplished only by dropping the foreign key constraint. That, and changing
the table structure, column names, and column types were successful. But, I
am not seeing the proper syntax to add a foreign key constraint back to the
two affected tables.

Pointer please.

Rich