Function isOperatorDef

src/compute-engine/boxed-expression/utils.ts:411–415

(
  def: BoxedDefinition | undefined
)

 * projection, applied recursively.
 *
 * The point is to avoid `.evaluate()`. Evaluating an `At` evaluates the picked
 * element too, which substitutes assigned symbol values: with `Y := 5`,
 * `At([Y, 2], 1).evaluate()` is `5`. Inside a held `Solve` equation that would
 * replace the very unknown being solved for. Projection just hands back the
 * operand.
 *
 * Without this, indexing into a computed list hid the unknown from the solver
 * exactly as a value-bound symbol did — `Solve(At([Y, 2], 1) = 5, Y)` returned