Function traceMatches

src/compute-engine/boxed-expression/solve.ts:2171–2183

(
      matches: RuleSteps,
      via?: { because: string; form: Expression }
    )

 * Examples: `(ln x)² − 4 → ln x = ±2 → {e², e⁻²}`,
 * `√(ln x) = ln√x → {1, e⁴}`, `e^{2x} − 3e^x + 2 → {0, ln 2}`.
 *
 * Returns `null` when no single generator captures every occurrence of `x`
 * (e.g. `sin x − tan x`, which has two independent generators), or when the
 * reduced equation has no roots.
 */
function solveByGeneratorSubstitution(
  expr: Expression,
  x: string,
  depth: number,
  trace?: RuleSteps
): ReadonlyArray<Expression> | null {
  if (depth >= 3) return null; // recursion backstop
  const ce = expr.engine;

  const normalized = expandLogPowers(expr);
  for (const g of collectGenerators(normalized, x)) {