Function isValueDef

src/compute-engine/boxed-expression/utils.ts:405–409

(
  def: BoxedDefinition | undefined
)

  if (resolved.every((op, i) => op === ops[i])) return expr;
  return expr.engine.function(expr.operator, resolved);
}

/**
 * Replace `At(List(e₁, …, eₙ), k)` by `e_k` — a purely *structural*
 * 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`,