Function selectFactors

src/compute-engine/rubi/rubi-utils.ts:3568–3581

(
  u: Expression,
  x: string,
  ce: ComputeEngine,
  free: boolean
)

// FunctionOfExponential family (IntegrationUtilityFunctions.m): recognize when
// an integrand u is a function of a single F^v (F constant, v linear in x) and
// rewrite F^v → x, for the master exponential-substitution rule 2.3#97:
//   Int[u] = v/D[v,x] · Subst[Int[FunctionOfExponentialFunction[u]/x, x], x, v]
// Rubi threads fluid $base$/$expon$/$exponFlag$; here the state is explicit.
// Hyperbolic heads count as exponentials (Sinh[w] ≡ (E^w−E^−w)/2), so the same
// machinery serves the Chapter-6 active-hyperbolic integrands.
// ---------------------------------------------------------------------------

const HYPERBOLIC_HEADS = new Set([
  'Sinh',
  'Cosh',
  'Tanh',
  'Coth',
  'Sech',
  'Csch',
]);

/** Does u contain a hyperbolic function head anywhere in its tree? Used by the