Foreign key references to non-primary key columns

Greetings!  I've noticed in the documentation that the sql standard
requires foreign keys to reference primary key/(or maybe just unique)
columns, but that postgresql does not enforce this.  Is this a feature
that is intended to persist, or a temporary deviation from the sql
standard?  The current postgresql behavior seems useful in cases where
one wants to update a foreign key to a value already in the original
table.  

Take care,
-- 
Camm Maguire                             camm@enhanced.com
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"The earth is but one country, and mankind its citizens."  --  Baha'u'llah