IF- statements in a rule's 'DO INSTEAD SELECT ...'- statement

Hello, all,

Can I insert IF- statements into a rule's DO INSTEAD- clause ? I have the
problem in a situation like :

create rule vprinsert as on insert to vpr_status
        do instead (DDDDD);

if I insert as DDDDD a genuine SELECT- statement (or, Alvaro, several select
statements), everything is OK; but if I start inserting IF- statements, it's
not OK any more (syntax errors). Don't I just know the exact syntax, or is
this simply impossible ?

Anyone any idea on how to resolve this ?

Regads,

Philippe Bertin.

P.S. To have an idea why I want to insert IF- statements into the rule,
please refer to my newsgroup posting of last Friday ...