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I think Pell urgently needs H3 and H4, because H4 is one of the the most useful headings.
I'd also like to see a line-end after each closing block level tag to make the code more readable.

Start by locating Pell's heading handling and the code that emits closing block-level tags. Confirm how existing heading levels and generated line endings are tested, then define completion as supporting H3 and H4 and adding the requested readability line endings without changing other output.

Written by the indexing model from the issue text.