Unique constraint over null values

Hi all,

I have a table like

CREATE TABLE a (type    CHAR (1) NOT NULL,data1    CHAR (16) NOT NULL,data2    CHAR (16) NULL
);

where type can be 's' for 'single' and 'x' for extended, so s should 
mean that there is only the type and data1 field, and x means that all 
fields are set.

How can I set a unique constraint like CHECK (type = 's' AND UNIQUE 
(type, data1)) OR (type = 'x' AND UNIQUE (type, data1, data2)) ?

The documentation says that 2 NULL values are different, so no unique 
constraint can be set over it.

Thanks in advance,

Thrasher