⍝ Fold (reduce) from the left:

    'abracadabra'{⍺~⍵⊃⍺} foldl 1 2              ⍝ rem all 'a's and all 'r's.
bcdb
    'abracadabra'{⍺~⍵⊃⍺} foldl 2 1              ⍝ rem all 'b's and all 'a's.
rcdr
    repl←subs⍨                                  ⍝ ⍺ with ⍵ replacement.

    pairs ← ('ny' 'kes')'iu'                    ⍝ from/to pairs

    'many a mickle'repl foldl pairs             ⍝ multiple letter replacement.
makes a muckle

    0 ,∘⊂⍨ foldl 2 5⍴⍳10                        ⍝ higher rank arrays.
┌──┬───────────────┐
│5 │┌─┬───────────┐│
│  ││4│┌─┬───────┐││
│  ││ ││3│┌─┬───┐│││
│  ││ ││ ││2│1 0││││
│  ││ ││ │└─┴───┘│││
│  ││ │└─┴───────┘││
│  │└─┴───────────┘│
├──┼───────────────┤
│10│┌─┬───────────┐│
│  ││9│┌─┬───────┐││
│  ││ ││8│┌─┬───┐│││
│  ││ ││ ││7│6 0││││
│  ││ ││ │└─┴───┘│││
│  ││ │└─┴───────┘││
│  │└─┴───────────┘│
└──┴───────────────┘

    scanl←{                ⍝ Dyadic Scan from the left (cf: ##.ascan).
        ⌽↑⍺⍺{
            (⊂(⊃⍵)⍺⍺ ⍺),⍵
        }/(⌽⍵),⊂⊂⍺
    }

    'hello' ⌽⍨ scanl 1 1 0 ¯1 ¯1
┌─────┬─────┬─────┬─────┬─────┬─────┐
│hello│elloh│llohe│llohe│elloh│hello│
└─────┴─────┴─────┴─────┴─────┴─────┘

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