Function substTrigNode

src/compute-engine/rubi/rubi-utils.ts:3466–3475

The substitution target trig node of `v` (the rule passes `trig(arg)/d`); * returns the bare trig node after dropping x-free factors, or null.

(
  v: Expression,
  x: string,
  ce: ComputeEngine
)

  if (op === 'Multiply') {
    const nonfree = u.ops.filter((o) => o.has(x));
    return nonfree.length <= 1 && nonfree.every((o) => easyDQ(o, x));
  }
  if (op === 'Add') return u.ops.every((o) => easyDQ(o, x));
  return false;
}

/** DerivativeDivides[y, u, x] — see block comment. Returns the x-free quotient
 * or the `False` symbol. */
function derivativeDivides(y: Expression, u: Expression, ctx: Ctx): Expression {
  const ce = ctx.ce;
  const x = ctx.x;
  const FALSE = ce.symbol('False');
  // Rubi: MatchQ[y, a_.*x /; FreeQ[a,x]] → False (degree-1 monomial, no