check?: a minor efficiency/debug clarity improvement

I have discovered that the nondeterminism introduced by MDO
clutters the logs.

As an aside -- would you be interested in a position->line/column mechanism,
if it were to introduce a dependency on cl-containers?  Or perhaps just an optional one?

The thing is, we need to map positions to line number/line-start positions,
for which I use a tree from cl-containers.

I've found that it helps debugging.