Holger,

HK> use any stored procedure as a function

I think this maps to the concept of stored function, which many
databases have. I too would find it useful.
AFAIK, a stored function:

- can have zero or more input parameters
- must have one and only one output parameters (the result)
- is defined with a specific keyword (create stored function ...)
- is then used the same way as a UDF

I guess in Firebird it could have additional output parameters other
than the result, but perhaps this would result in messier selects.
Ciao
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