constraints for ensuring relationships

Hi,

I wonder if I can use constraints to ensure some special integrity.

Suppose there is a table "Type" of item types:

CREATE TABLE "Type"
("index"    SERIAL PRIMARY KEY,"name"     TEXT,"multiple" BOOL
);

Suppose there is a table "Item" like this:

CREATE TABLE "Item"
("index"    SERIAL PRIMARY KEY,"type"     INT REFERENCES "Type","data"     TEXT
);

Supose there is a table "SubItem" that contains special details about our 
items:

CREATE TABLE "SubItem"
("index" SERIAL PRIMARY KEY,"Item"  INT REFERENCES "Item","Data"  TEXT
);

And now the constraint:

Each Item should be allowed to have more than one subitem, if and only if its 
type allows for multiple instances, i. e. the type the item is linked to must 
have 't' in its field 'multiple'.

Can I attach constraints to some of the tables (e. g. "SubItem") that ensure 
this?

Thanks a lot!

Markus