How to implement a 'default' flag that can only be set on a single row

For example, with a table similar to this:

create table foo(bar int identity, chk char(1) check (chk in('Y', 'N')));

It doesn't matter if the flag is implemented as a char(1), a bit or whatever. I just want to be able to enforce the constraint that it can only be set on a single row.