Whether the author wrote a block in this slot at all, readable or not.
null from readDeclarationSlot answers two different questions with
one word — "nothing was written here" and "something was written and this
module rejected it" — and spec §6(b)'s precedence turns on which: a block
that was written and rejected is still the author saying something about
this series, so the reading falls through to the heuristic rather than on
to a chart-level declaration meant for the series that said nothing.
Shares its guard with readDeclarationSlot rather than restating it,
so the two cannot come to disagree about what a slot is.
Whether the author wrote a block in this slot at all, readable or not.
nullfrom readDeclarationSlot answers two different questions with one word — "nothing was written here" and "something was written and this module rejected it" — and spec §6(b)'s precedence turns on which: a block that was written and rejected is still the author saying something about this series, so the reading falls through to the heuristic rather than on to a chart-level declaration meant for the series that said nothing.Shares its guard with readDeclarationSlot rather than restating it, so the two cannot come to disagree about what a slot is.