I plan to tweak the sample implementation so that if there's an error,
it will try to resync by reading to find an un-indented line.

It currently tries to resync by reading to find a _blank_ line.
But this causes it to sometimes skip lines that are obviously the
start of a new sweet-expression, making the current
behavior suboptimal.  (If there's a collecting list
it could be at the wrong level, but that'd be true regardless.)

The spec doesn't require any particular resync behavior.
I think that's fine; this is more a quality-of-implementation issue.
But I think the sample implementation should behave nicely, since
that's how many people will first encounter this notation.

--- David A. Wheeler