pg_shadow in a constraint

Dear All!

in my system i would like to use the pg user's usesysid as a key in my
tables.
But it is not allowed (that was many times described here and in the
documentation)
But what can i do to get my tables and data to be consistent with the
system catalogue?
Could you please supply me with a right URL or at least a direction
where to search for solution?

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Best regards,
Anton