Given an array of ints, compute recursively if the array contains a 6. We'll use the convention of considering only the part of the array that begins at the given index. In this way, a recursive call can pass index+1 to move down the array. The initial call will pass in index as 0.
array6([1, 6, 4], 0) → true
array6([1, 4], 0) → false
array6(, 0) → true
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