Isolation and study of a Saponin (Echinocystic acid-3-o-α-l–Rhamnopyranosyl (1→5)–o–β-d–xylofuranosyl (1→5)–o–β-d–Arabinofuranosyl (1 → 4)–o–β-d–Glucopyranoside) from the leaves of Clematis nepaulensis D.C.