RML004 — an index outside the array

at(i) accepts -length … length - 1, negative counting from the end, so at(-1) is the last element. insert(i, …) accepts one more — length itself, which appends, and push says that more plainly.

A warning, not an error: the index is the code's, but the length is the data's, and an array that came back shorter than expected is a normal thing for it to do.

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