dropping a table constraint

I needed to add a FK constraint to a table where one was forgotten. I found an add constraint command and used it:

ALTER TABLE file_info add foreign key (al_fk) references arraylayout(al_pk);

However, I wanted to name the constraint. It now looks like this:

$1 FOREIGN KEY (al_fk) REFERENCES arraylayout(al_pk) ON UPDATE NO ACTION ON DELETE NO ACTION

I have tried using the drop command so I could recreate it with a name but the drop command is not working.
I am on version 7.3.4.

can anyone tell me what I am doing wrong? I could dump and restore the entire db but do not want to if it's not necessary.
Thanks
Jodi