Function isCheaper

src/compute-engine/boxed-expression/simplify.ts:234–256

(
  oldExpr: Expression,
  newExpr: Expression | null | undefined,
  costFunction?: (expr: Expression) => number
)

        // Mark it as a SHIELD — see `withValueShield`, the other site.
        markShieldDeclaration(shadowScope, n);
      } catch {
        /* leave this symbol unshadowed */
      }
    });
    steps = simplify(expr, options);
  } finally {
    ce.popScope();
  }
  // The shadow bindings are dead now that the scope is popped, but a result
  // that could not fully reduce still mentions the shielded symbols and would
  // carry those dead bindings out to the caller — where they compare unequal
  // to the caller's own symbols. Same fix as `withValueShield`; this is the
  // second site of the same pattern, reached by the public `.simplify()`.
  //
  // Only the FINAL step's value escapes as the caller's result; the
  // intermediate steps exist for `explain()`, which serializes them by name.
  // Rebinding only the last value keeps the fix off the O(steps × tree) path.
  const last = steps.at(-1);
  if (last === undefined) return steps;
  const rebound = rebindEscaping(last.value, shadowScope);
  if (rebound === last.value) return steps;
  return [...steps.slice(0, -1), { ...last, value: rebound }] as RuleSteps;
}

export function simplify(
  expr: Expression,