opaque in sql function

In the doc I read it's possible to use 'opaque' as rtype in sql
functions, but when I try to create a sql function with return opaque I

got

the message 'ProcedureCreate : sql functions cannot return type

"opaque"...

When I declare tye function as 'plpgsql' return opaque is ok, but on exec
I got the message "Invalid TypeIdTypeRelId - oid -0"
I use Pg 7.0.2 with redhat 6.1.

OPAQUE datatype is used only by internal functions that make calls to other
functions. It is not datatype actually, but some kind of instruction to make
no assumptions about return type. Actual meaning depends on what function is
it - trigger, constraint etc. You cannot use OPAQUE functions in queries.

Alex Bolenok.