primary key as subtype discriminator

Hello,

I have something like:

TABLE A
a PK

TABLE B
b PK

TABLE AB (many-to-many table)
a, b PK

TABLE AB-1 (AB subtype)
a, b PK

TABLE AB-2 (AB subtype)
a, b PK

Can I use the attribute "a" (part of PK) as subtype discriminator?

Thanks,
philip