ahead thru messes up errors again

[homonoidian]

Before we did delta analysis. This was possible (and is still possible & done) because inject/eject modify the size of the block, which we use as some sort of "cursor position compensation trick". ahead thru only moves the cursor, which makes it very fast (it's basically an integer increment), but that complicates the compensation because we don't know whether cursor movement was good (e.g. in Engine, that helps us find the opened word) or bad (made by thru and therefore out-of-place in terms of call/open trace).

Since ahead thru is best for speed, we'll need to adapt the rest of the infrastructure instead of just getting rid of it.