Every token must be locatable in every cited ref in every source.
Affects: @pos
@val
@rgx
<tok>
<from-tok>
<through-tok>
Used by template ŧ tan:expand-reassigns
, ŧ tan:class-2-expansion-terse
tan:class-2-expansion-terse-for-validation, ŧ tan:mark-tok-pos
, ŧ tan:mark-dependencies-pass-2-for-validation
.