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Map {::  _ 1 _ Fetch

{::y has the same boxing as y and its elements are the paths to each leaf (each open array).
 		   x{::y fetches a subarray of y according to path x ; the selection at each level is based on { and, except at the last level, must result in an atom.
 

Map and Fetch can be modeled as follows:
   cat  =: { @: (i.&.>) @: $
   mapp =: 4 : 'if. L. y. do. (<"0 x.,&.><"0 cat y.) mapp&.> y.   
                 else. >x. end.'
   map  =: a:&mapp
   fetch=: >@({&>/)@(<"0@|.@[ , <@]) " 1 _
The following phrases illustrate the use of Map and Fetch:
   ] y=: 1 2 3;4 5;i.4 5
+-----+---+--------------+
|1 2 3|4 5| 0  1  2  3  4|
|     |   | 5  6  7  8  9|
|     |   |10 11 12 13 14|
|     |   |15 16 17 18 19|
+-----+---+--------------+
  (2;_1 _1){::yThe number 19
  (_1;3 4) {::yThe number 19
  {::yPaths to each open array
  {::cat L: 0 yPaths to each open scalar
   ] t=: 5!:2 <'fetch'         An array with an interesting structure
+------------------------------------------------------+-+---+
|+-----------------+-+--------------------------------+|"|1 _|
||+-+-+-----------+|@|+--------------------+-+-------+|| |   |
|||>|@|+-------+-+|| ||+--------------+-+-+|,|+-+-+-+||| |   |
||| | ||+-+-+-+|/||| |||+-------+-+--+|@|[|| ||<|@|]|||| |   |
||| | |||{|&|>|| ||| ||||+-+-+-+|@||.|| | || |+-+-+-+||| |   |
||| | ||+-+-+-+| ||| |||||<|"|0|| |  || | || |       ||| |   |
||| | |+-------+-+|| ||||+-+-+-+| |  || | || |       ||| |   |
||+-+-+-----------+| |||+-------+-+--+| | || |       ||| |   |
||                 | ||+--------------+-+-+| |       ||| |   |
||                 | |+--------------------+-+-------+|| |   |
|+-----------------+-+--------------------------------+| |   |
+------------------------------------------------------+-+---+
  (0;2;0;0;0){:: t Fetch the subarray corresp. to <"0 in t
  (0;2;0;0;0;_1){:: t Fetch the 0 in that
  t ,&< L: 0 1 {:: t Label each leaf with its path
  < S: 0 t The boxed leaves of t
  < S: 1 {:: t The boxed paths of t
  t ,&< S: 0 1 {:: t A 2-column table of leaves and paths
  # 0: S: 0 t The number of leaves in t



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