Create Assertion -- Question from a newbie

Sorry if my question is a little off topic.

I am reading my new "SQL for Smarties" book side by side with the PostgreSQL 8.1 manual.  I
noticed that this particular feature is not included in PostgreSQL.  Some of the achieve threads
mostly discuss that this feature is currently not supported. My understanding is that Assertions
place constraints upon data spanning multiple related tables. 

Is the Assertion feature slated to be added in the future?  (Perhaps rolled up in a more
generalized "TO-DO" item?)

Would this feature add functionality that can not be achieved by other means? (i.e. alternative
schema definitions or triggers?) Or does it merely provide a redundant means to constrain data,
and thereby not warrant addition into the features of PostgreSQL?

Regards,

Richard Broersma Jr.