Calling a trigger from a trigger

I have a number of tables that have a group of fields in common.
I want to process theses fields in a before trigger (before insert or
update), but each of the tables have before triggers for processing of
other non-common data fields.

It seems that from a pl/pgsql trigger I cannot call another trigger (the
one that would be common to all) and if I just attach the common trigger as
a second trigger to each table then (according to the docs) there is no
guarantee of the execution order (which I need ... each table requires the
common stuff to be processed before the non-common stuff).

Is there a way to achieve this requirement without duplicating the code
from the common-part in each of table triggers?

Thanks, in advance, for any thoughts
Mike.

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Mike Finn
mike(dot)finn(at)tacticalExecutive(dot)com