0 - Multiplicity (count character)
    'h'(+/=)'fhqwhgads'

    1 - Trapeze Part (palindrome)
    (⊢≡⌽)'racecar'

    2 - chars which appear more than once
        (not too happy with my Key ⌸ ones)
    {k←{⍺,⍨1<≢⍵}⌸⍵ ⋄ k[;1]/k[;2]} 'applause'

    3 - reordered letters
    'teapot'{⍺[⍋⍺]≡⍵[⍋⍵]}'toptea'
    ≡/(⊂∘⍋⌷⊢)¨'teapot' 'toptea'

    4 - chars which appear once
    {k←{⍺,⍨1=≢⍵}⌸⍵ ⋄ k[;1]/k[;2]} 'somewhat heterogenous'

    5 - musical chars
    'barfoo' {∨/⍺≡⍤1⊢⍵⌽⍤0 1⍨⍳≢⍵} 'foobar'

    6 - sort strings by length, ascending
    {⍵[⍋≢¨⍵]} 'books' 'apple' 'peanut' 'aardvark' 'melon' 'pie'
    ((⊂∘⍋(≢¨⊢))⌷⊢) 'books' 'apple' 'peanut' 'aardvark' 'melon' 'pie'

    7 - Most frequent character
    {k←{⍺,≢⍵}⌸⍵ ⋄ ⊃k[;1][⍋k[;2]]} 'abdbbac'

    8 - reverse words
    {⊃{⍺,' ',⍵}/⌽¨' '((~=)⊆⊢)⍵} 'a few words in a sentence'

    9 - compress
    1 0 0 1 0 1/'foobar'

    10 - expand
    1 0 0 1 0 1 {'_'@(⍸~⍺)⍺\⍵} 'fbr'

    11 - Consonants
    {'_'@(⍸'AEIOUYaeiouy'∊⍨⍵)⊢⍵} 'FLAPJACKS'

    12 - Consonants Rdx
    {⍵/⍨~'AEIOUYaeiouy'∊⍨⍵} 'FLAPJACKS'

    13 - Word replace
    ⍸'a few words in a sentence' {idx←⍸⍵⍷⍺ ⋄ idx,←idx+¨⍳¯1+≢⍵ ⋄ 'x'@idx⊢⍺} 'words'

    14 - Permutations
    ? a recursive one shouldn't be so hard, but..
    {0=≢⍵:'' ⋄ ... } 'xyz'

    non-recursive, much harder:
    https://code.jsoftware.com/wiki/Doc/Articles/Play202