As headlines are always lvl (indent) 0 I thought it would be clever to abuse
the lvl field to store the headline lvl. Well, here we are - it wasn't clever.
List items only end when their parent ends or they run into something that's
not indented enough - everything else becomes part of the list item. Abusing
the token.lvl field (indent) for the headline lvl means headlines look indented
to the list item parsing logic - i.e. they become part of the list item if the
headline has a high enough lvl. That should never happen - so let's get rid of
the hack and (re-)calculate the headline lvl when we need it.
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