as i spend time around it, and see things, i remember it a little better. i think i actually resolved the assertion on line 42, by accommodating data with zero-lengthed branches. this separates the concerns.
then i think i ran into the assertion on line 61 as a remaining issue. so i could think of the problem space as relatively small: - index of zero size being generated. data may still be valid. maybe it just makes a branch when there is no new data. - iterating over the data is passing by data that exceeds the length of the stream. this could be an issue with the data index, or it could be an issue with the algorithm that iterates. i'm suspecting the algorithm, but there could also be a mistake in the data index too.