of←{                            ⍝ Pick of F of G of H ... arg.
    indx rslt←⍺⍺{               ⍝ 'countdown' and received result.
        2=⎕NC'⍺⍺':⍺⍺ ⍵          ⍝ left leaf of tree: pick-index and arg.
        'int'⍺⍺ ⍵               ⍝ intermediate level: recur left.
    }⍵

    next←(⍵⍵⍣(⎕IO=indx))rslt    ⍝ apply ⍵⍵ if at right level.

    ⍺←'top'                     ⍝ missing ⍺ => top level.
    ⍺≡'top':next                ⍝ top level: return result.
    (indx-1)next                ⍝ otherwise: index and rslt.
}

code_colours

test script

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