I mistyped a start clause in a tumbling window expression using "where" instead of "when"

for tumbling window $w in (1 to 20) start at $s where $s mod 3 = 1 return <window>{$w}</window>

see https://fiddle.basex.org/?share=%28%27query%21%27for+tumbling+-*w+in+%7B1+to..., but I didn't get an error but a result

<window>1</window> <window>4</window> <window>7</window> <window>10</window> <window>13</window> <window>16</window> <window>19</window>

Shouldn't that query give a syntax error that "when" is expected instead of "where"?

Or is that a new XQuery 4 feature that the "when" clause can be omitted and the above is kind of parsed/executed as

for tumbling window $w in (1 to 20) start at $s where $s mod 3 = 1 return <window>{$w}</window>

?