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Previously, just referencing the next element in the sequence (and so constructing it, as an undefined element) would allow you to skip defining an element without turning the sequence into a map. E.g:

node[0] = "foo"; // sequence of size 1
node[1]; // sequence of size 1, with an undefined element at 1
node[2] = "bar"; // FIX: should be map of size 2 (since there's no element at index 1)