Commented by: @sim1984

That's a moot point. According to the documentation of such operator precedence
1. Concatenation
2. Arithmetic operators
3. Comparison operators
4. Logical operators

However, the documentation does not say anything about the priority of the predicate IS [NOT] {TRUE | FALSE | UNKNOWN}

I think that the predicate IS should be calculated before the operator "=". Thus second expression should be equivalent to:
NOT (FALSE = (true IS NOT UNKNOWN)) i.e.
NOT (FALSE = TRUE) -> true

It would be reflected in the documentation what the priority of predicates IS [NOT] {TRUE | FALSE | UNKNOWN}

Commented by: @pavel-zotov

On WI-T4.0.0.132:

select ((not false) is true) is unknown from rdb$database; -- false; OK, expected
select (not false is true) is unknown from rdb$database; -- false; OK, expected

But:

select not false is true is unknown from rdb$database; -- true -- WHY ??

Evaluate from left to right:

1) NOT false ==> TRUE(a),
2) TRUE(a) is TRUE ==> TRUE(b)
3) TRUE(b) is UNKNOWN ==> FALSE

Evaluate from right to left:
1) TRUE is UNKNOWN ==> FALSE(a)
2) NOT FALSE is FALSE(a) ==> should be FALSE for any way of evaluation:
2a) NOT(false is false) ==> not (true) ==> false
or
2b) (not false) IS false ==> (true) IS false ==> false.

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PS. Fully agree with:

> Simonov Denis:
> It would be reflected in the documentation what the priority of predicates IS [NOT] {TRUE | FALSE | UNKNOWN}

Commented by: @sim1984

Pavel Zotov,

see precedence for operator types above

Logical operators calculated after Comparison operators
IS - comparation operator
NOT - logical operator

not false is true is unknown => not ((false is true) is unknown)

>> "I think that the predicate IS should be calculated before the operator ="
I'm not sure it's right. What about the priorities of operators said standard?

Precedence in Postgresql
http://www.postgresql.org/docs/9.0/static/sql-syntax-lexical.html#SQL-PRECEDENCE-TABLE
but
http://www.postgresql.org/docs/9.5/static/sql-syntax-lexical.html#SQL-PRECEDENCE-TABLE

Commented by: @asfernandes

> select not false is true is unknown from rdb$database; -- true -- WHY ??

Result matches PgSQL too.

For the precedence documentation, we do not have a readme for that, so I believe a doc. ticket should be created pointing what doc. is outdated.