Function updateDef

src/compute-engine/boxed-expression/utils.ts:417–445

(
  ce: ComputeEngine,
  name: string,
  def: BoxedDefinition,
  newDef:
    | Partial<OperatorDefinition>
    | BoxedOperatorDefinition
    | Partial<ValueDefinition>
    | BoxedValueDefinition
)

 *
 * 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
 * `[]`, i.e. "proven no solutions".
 *
 * Only a literal `List` with a literal integer index in range is reduced;
 * indices are 1-based and a negative index counts from the end, matching `At`.
 */
export function reduceStructuralIndex(expr: Expression): Expression {
  if (!isFunction(expr)) return expr;

  const ops = expr.ops;
  const reduced = ops.map(reduceStructuralIndex);
  const self = reduced.every((op, i) => op === ops[i])
    ? expr
    : expr.engine.function(expr.operator, reduced);

  if (!isFunction(self, 'At') || self.nops !== 2) return self;

  const list = self.op1;
  if (!isFunction(list, 'List')) return self;

  const index = self.op2;
  if (!isNumber(index)) return self;
  // A complex index (`1 + 2i`) is not a valid list position: decline rather
  // than silently projecting on its real part.
  if (index.im !== 0) return self;
  const k = index.re;
  if (!Number.isInteger(k) || k === 0) return self;

  const n = list.nops;
  const i = k > 0 ? k : n + k + 1;
  if (i < 1 || i > n) return self;