Unique indices and nulls

I see in the documentation that a unique index does not place any
constraints on the number of nulls in a table. In my case, I have a
table with has four columns (plus other data) where that combination
of 4 columns should be unique.

(catalog, entry, suffix, component)

forms a unique tuple. In most (but not all) cases, (catalog, entry)
is unique and suffix and component will both be null. In those cases,
it is common to have an entry with (catalog, entry, null, null), as
well as multiple entries with (catalog, entry, suffix, component).

But there should never be more than one entry with (catalog, entry,
null, null).

Is there any way I can enforce this? Am I going to have to write a
trigger to check for duplicates?

roland
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