There can be only one

Consider a table of providers, for which one is the default. For example,
payment providers:

CREATE TABLE payment_via (
id uuid PRIMARY KEY,
provider text NOT NULL,
keys hstore NOT NULL DEFAULT ''
);

Here we store together the name of the provider — medici, paypal — with
access tokens needed to use a certain payment account. How shall we store
which one is the default? Ideally, we’d be able to ensure there is *but one*
default.

CREATE TABLE payment_via (
id uuid PRIMARY KEY,
provider text NOT NULL,
keys hstore NOT NULL DEFAULT '',
is_default boolean NOT NULL DEFAULT FALSE
);

How shall we state the constraint? The obvious thing would seem to be:

CREATE TABLE payment_via (
id uuid PRIMARY KEY,
provider text NOT NULL,
keys hstore NOT NULL DEFAULT '',
is_default boolean NOT NULL DEFAULT FALSE,
EXCLUDE (is_default USING AND)
);

However, this is a syntax error. There is always:

CREATE TABLE payment_via (
id uuid PRIMARY KEY,
provider text NOT NULL,
keys hstore NOT NULL DEFAULT '',
is_default boolean NOT NULL DEFAULT FALSE,
EXCLUDE (is_default USING =) WHERE (is_default)
);

but this seems awkward and I was hoping there was some way to use AND as an
operator.
​