SPI returns error for SAVEPOINT

A plandl (language handler for Andl) function is called as follows:

BEGIN;
SELECT plandl_compile($1); // argument is Andl code
COMMIT;

Inside:

>SPI_exec("BEGIN",...) returns error SPI_ERROR_TRANSACTION. As expected.
>SPI_exec("SAVEPOINT xyz",...) returns error SPI_ERROR_TRANSACTION. Not
expected.

Is it not possible to set a SAVEPOINT inside a function? Or am I missing
something?

Regards
David M Bennett FACS

Andl - A New Database Language - andl.org