Feature request - function-based deferrable uniques.

For now Postgres able to create deferrable uniques with following syntax:

...
and table_constraint is:

[ CONSTRAINT constraint_name ]
{ UNIQUE ( column_name [, ... ] ) index_parameters | PRIMARY KEY ( column_name [, ... ] ) index_parameters | CHECK (
expression) | FOREIGN KEY ( column_name [, ... ] ) REFERENCES reftable [ ( refcolumn [, 
 
... ] ) ]   [ MATCH FULL | MATCH PARTIAL | MATCH SIMPLE ] [ ON DELETE action ] [ ON 
UPDATE action ] }
[ DEFERRABLE | NOT DEFERRABLE ] [ INITIALLY DEFERRED | INITIALLY IMMEDIATE ]

So, deferrable uniques now can be based on column/columns list only. It will 
be very useful if there will be possibility to specify functions in this list. 
Is it possible? 

Regards, 
Dmitry