I think the test for nested aggregate functions is incorrect.
The following statement is allowed:

SELECT
SUM(t1.COL1 - (SELECT t2.COL1 FROM TABLE2 t2 WHERE t2.COL2 = ?))
FROM
TABLE1 t1

This statement is not (returns 'Nested aggregate functions are not
allowed'):

SELECT
SUM(t1.COL1 - (SELECT SUM(t2.COL1) FROM TABLE2 t2 WHERE t2.COL2 = ?))
FROM
TABLE1 t1

In both cases the SELECT on TABLE2 is standalone, and does not require
any nested aggregation on the original stream.

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