Multiple anyelement types

I'm working on improvements of orafce.
http://pgfoundry.org/projects/orafce

I found postgres supports only one type of anyelement at one time
when I added nvl2() and decode(). I'd like to use multiple types of
anyelement something like:
 template < typename Expr, typename Ret > CREATE FUNCTION nvl(Expr, Ret, Ret) RETURNS Ret
 template < typename Expr, typename Ret > CREATE FUNCTION decode(Expr, Expr, Ret, ..., Ret) RETURNS Ret

I don't mean to propose the above C++-like syntax, but such feature
is important to develop a generic porting tool. Two independent
anyelements are enough for me, but three or more might be better
for general use.

What syntax is suitable for postgres? Comments welcome.

Regards,
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ITAGAKI Takahiro
NTT Open Source Software Center