Set operations with three or more branches drop every middle branch

[gmr]

Every style drops the middle branch of a UNION / INTERSECT / EXCEPT chain with three or more branches. Confirmed on the published v1.4.0 and on main.

SELECT 1 UNION ALL SELECT 2 UNION ALL SELECT 3;
-- formats to
SELECT 1 UNION ALL SELECT 3;

SELECT 1 UNION SELECT 2 EXCEPT SELECT 3 INTERSECT SELECT 4;
-- formats to
SELECT 1 EXCEPT SELECT 3 INTERSECT SELECT 4;

The output parses and runs, and returns different rows.

Cause

A UNION B UNION C parses left-nested: ((A UNION B) UNION C). collect_clauses_recursive() recurses into the left select_clause, sets clauses.set_op to UNION B, then returns to the outer level and sets clauses.set_op to UNION C, replacing it. SelectClauses holds one set operation, so B is lost.

Right-nested chains (A UNION (B INTERSECT C), which is how INTERSECT's higher precedence parses) are not affected, because the right side is collected recursively with its own set_op.

Fix

Append the outer operation to the end of the existing chain instead of replacing it. The formatter emits the branches as a flat sequence in source order, and PostgreSQL applies the same precedence when it parses that sequence, so the chain's grouping does not need to be kept.

Found by the token-loss guard added in #59; part of #58.