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Jelly has an "untruth" atom: Ṭ. This takes a non-empty array of positive integers and returns a Boolean array with 1s at the indexes in the input. For example:
[1,3,5,6]Ṭ ⁼ [1,0,1,0,1,1]
[5]Ṭ ⁼ [0,0,0,0,1]
[2,1,1,2]Ṭ ⁼ [1,1]
[5,4,3]Ṭ ⁼ [0,0,1,1,1]
[1]Ṭ ⁼ [1]
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