The subtree to evaluate when fragment is null.
The run's text, to evaluate as a standalone expression (=<fragment>), or null to evaluate
evalIndex on the whole formula instead. Set only when the run is a valid expression that
ISN'T a subtree of the formula — see the note on evalTargetForSelection.
Last token of the resolved run (inclusive).
First token of the resolved run (inclusive) — what the host outlines.
Character span of [lo, hi] in the text — what F9 replaces.
What a text selection resolves to — see evalTargetForSelection.