The logical operators in an EventFilter where-clause use three distinct truth values, TRUE, FALSE and NULL (cf. Part 4, Table 127 and 128).

TRUE and FALSE are Booleans whereas NULL is indicated by an empty variant.
Part 4 further describes casting to Boolean.
Implicit casting is only possible from String to Boolean (e.g. "false" -> FALSE).
Explicit casting is possible from any numerical type to Boolean (e.g. x != 0 -> TRUE).

In our implementation, we are unsure of the casting rules when calling a logical operator with a non-Boolean.
The standards speaks of an "operand that resolves to a Boolean".

NOT TRUE -> FALSE
NOT NULL -> NULL
NOT "true" -> ???
NOT 1 -> ???
NOT ns=1;i=123 -> ???

Does resolving to a Boolean include automatic casting and also casting of numericals (explicit).
Also, a NULL value is considered an "empty value". Does that mean any non-empty value is non-NULL and resolves to TRUE?

The EventFilter operators that need clarification are AND/OR/NOT/ISNULL.