CHECK clause doesn't work with CASE clause

Hi folks,

I have a problem with a CHECK clause that doesn't seem to work properly. The
CREATE TABLE statement looks like this:

CREATE TABLE epilepsy_information (
[...]
epilepsy_class Char
CHECK (epilepsy_class IN ('f', 'g', 'n')),
focus Integer REFERENCES focus(id)
CHECK (
CASE
WHEN ((focus <> NULL) AND (epilepsy_class = 'f')) THEN
TRUE
WHEN ((focus <> NULL) AND (epilepsy_class IN ('g',
'n'))) THEN FALSE
END),
[...]
);

It shouldn't be possible to insert a value into focus when epilepsy_class
has one of the values 'g' or 'n'. But it is. Can anyone help?

Thanx sincerefully,
Joerg