The concrete exercise: every reference to a declared variable substituted. The same object as the input when it declares no variables.
The tolerance of every accept entry that is exactly one reference to
a variable with a tolerance, keyed by the entry's substituted text.
A partial reference ("The answer is {{sum}}") grades as text.
The value per declared variable, rounded to its display precision: the shape to persist with an attempt so a review can replay it.
What resolveExerciseVariables returns.