Transforming IN (...) to ORs, volatility

We sometimes transform IN-clauses to a list of ORs:

postgres=# explain SELECT * FROM foo WHERE a IN (b, c);                      QUERY PLAN
------------------------------------------------------ Seq Scan on foo  (cost=0.00..39.10 rows=19 width=12)   Filter:
((a= b) OR (a = c))
 
(2 rows)

But what if you replace "a" with a volatile function? It doesn't seem 
legal to do that transformation in that case, but we do it:

postgres=# explain SELECT * FROM foo WHERE (random()*2)::integer IN (b, c);
       QUERY PLAN 
 


-------------------------------------------------------------------------------------------------
------------------- Seq Scan on foo  (cost=0.00..68.20 rows=19 width=12)   Filter: ((((random() * 2::double
precision))::integer= b) OR 
 
(((random() * 2::double precision))::integer = c))
(2 rows)

I tried to read the SQL spec to see if it has anything to say about 
that, but I couldn't find anything. My common sense says that that 
transformation is not legal.

--   Heikki Linnakangas  EnterpriseDB   http://www.enterprisedb.com