Dan Bron wrote:
DB> As an intersting aside,  &]  is sort of an "identity adverb", in that 
DB> (u y) -: u&] y  and  (x u y) -: x u&] y  .

Almost.  The ranks of  u (? ? ?)  vs  u&] (_ _ _)  may differ.
The effects become visible only within suitable contexts, for example

   1  <@  +    2 3
+-+-+
|3|4|
+-+-+
   1  <@(+&])  2 3
+---+
|3 4|
+---+

                                                        Martin
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