Function sumTermsX

src/compute-engine/rubi/rubi-utils.ts:1587–1594

flatten a sum into its terms (Add/Subtract/Negate normalized)

(u: Expression)

/** structural monomial split u = coef·x^exp (coef and exp x-free; exp may
 * be symbolic). No expansion — mirrors Mathematica's literal matching of
 * a_.*x_^n_. terms over CE canonical form (Divide/Negate/Sqrt present). */
function monoPartsX(
  u: Expression,
  x: string
): { coef: Expression; exp: Expression } | null {
  const ce = u.engine;
  if (!u.has(x)) return { coef: u, exp: ce.Zero };
  if (u.symbol === x) return { coef: ce.One, exp: ce.One };
  const ops = u.ops;
  if (!ops) return null;