I've lost track of the number of times I have tried to make dyadic ;: work.
I read it, understand it (I think), but I can't get it to work. The general
description has m, the input mapping as a list of boxes, each box
containing the items to be mapped to the same number. So, in an attempt to
figure out what is going on I converted the example in help to that form
and get an error.

Given sj, mj and y as defined in help:

   (0;sj;mj) ;: y
+---+--+-+--+---+-+-+-+-+-+---+-+---+-+
|sum|=.|(|i.|3 4|)|+|/|.|*|0j4|+|pru|4|
+---+--+-+--+---+-+-+-+-+-+---+-+---+-+

So far, so good. Convert mj to the boxed form (display shortened to fit
e-mail):

   mj2=:(/:~.mj){mj</.a.
   ,.50{.&.>mj2
+--------------------------------------------------+
|    ┌┬┐├┼┤└┴┘│─!"#$%&()*+,-/;<=>?@|
+--------------------------------------------------+
|                                                  |
+--------------------------------------------------+
|ACDEFGHIJKLMOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz|
+--------------------------------------------------+
|N                                                 |
+--------------------------------------------------+
|B                                                 |
+--------------------------------------------------+
|0123456789_                                       |
+--------------------------------------------------+
|.                                                 |
+--------------------------------------------------+
|:                                                 |
+--------------------------------------------------+
|'                                                 |
+--------------------------------------------------+

Test it to see if it is ok.

   my=. (y i.~;m) { (#m),~(#&>m)#i.#m   NB. From help
   y,:,":"0 my
sum=. (i.3 4)+/ .*0j4+pru 4
222061026515000160525022215

Looks good, but

   (0;sj;mj2) ;: y
|length error
|   (0;sj;mj2)    ;:y

The problem is not with y, but something about mj2.

   (0;sj;mj2)&;:
|length error
|       (0;sj;mj2)&;:

As I understand the description in help, this should work.

So, what am I missing?
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