Hah, I should of scrolled down further. I think we should still add isASCII. For determining if a byte is a leading byte of a multi-code-unit sequence, it can be done with !isASCII && !isContinuation. I don't know if we'd still want a isLeadByte and if so, what we should name it. isLead makes it feel like it's made out of Pb, isLeading is much better but at odds with UTF16.isLeadSurrogate, and as @bobergj pointed out, it might not be worth adding at this point.
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