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http://heycam.github.io/webidl/#idl-sequence [[ sequence<T> ]] When we know the length of the array we want to return, e.g. for an iterator value, would it help if we enumerated the length and types? E.g. https://xhr.spec.whatwg.org/#formdata would probably return an array for each iterator value of length two where the first value is a ScalarValueString and the second a FormDataEntryValue.
This still seems somewhat useful, also on the accepting side only wanting to accept two items in a list and not three.