The " and / primitives are rather important.
This might help understand how they work.
F=.<@;
'left' F 'right'
+------------+
|+----+-----+|
||left|right||
|+----+-----+|
+------------+
1 2 3 (F"0 _)/ 40 50 60 70 80
+------------------+------------------+------------------+
|+-+--------------+|+-+--------------+|+-+--------------+|
||1|40 50 60 70 80|||2|40 50 60 70 80|||3|40 50 60 70 80||
|+-+--------------+|+-+--------------+|+-+--------------+|
+------------------+------------------+------------------+
1 2 3 (F"0 0)/ 40 50 60 70 80
+------+------+------+------+------+
|+-+--+|+-+--+|+-+--+|+-+--+|+-+--+|
||1|40|||1|50|||1|60|||1|70|||1|80||
|+-+--+|+-+--+|+-+--+|+-+--+|+-+--+|
+------+------+------+------+------+
|+-+--+|+-+--+|+-+--+|+-+--+|+-+--+|
||2|40|||2|50|||2|60|||2|70|||2|80||
|+-+--+|+-+--+|+-+--+|+-+--+|+-+--+|
+------+------+------+------+------+
|+-+--+|+-+--+|+-+--+|+-+--+|+-+--+|
||3|40|||3|50|||3|60|||3|70|||3|80||
|+-+--+|+-+--+|+-+--+|+-+--+|+-+--+|
+------+------+------+------+------+
1 2 3 (F"_ 0)/ 40 50 60 70 80
+----------+----------+----------+----------+----------+
|+-----+--+|+-----+--+|+-----+--+|+-----+--+|+-----+--+|
||1 2 3|40|||1 2 3|50|||1 2 3|60|||1 2 3|70|||1 2 3|80||
|+-----+--+|+-----+--+|+-----+--+|+-----+--+|+-----+--+|
+----------+----------+----------+----------+----------+
1 2 3 (F"_ _)/ 40 50 60 70 80
+----------------------+
|+-----+--------------+|
||1 2 3|40 50 60 70 80||
|+-----+--------------+|
+----------------------+
1 2 3 (F)/ 40 50 60 70 80
+----------------------+
|+-----+--------------+|
||1 2 3|40 50 60 70 80||
|+-----+--------------+|
+----------------------+
----------------------------------------------------------------------
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