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The "at" declarations do not make sense and lead to type errors, as $x and $y are bound to integer values: start at $x when $x[self::dt] end at $y next $z when $y[self::dd] and $z[self::dt] The correct query seems to be: for tumbling window $w in $seq/doc/* start $x when $x[self::dt] end $y next $z when $y[self::dd] and $z[self::dt] return ...
I agree and I have changed the current working draft accordingly.