例句：He lay at full length upon his stomach，his head resting upon his left forearm．他的脊背朝天，四肢伸展，頭枕著左臂，直挺挺地趴伏著。　　

句中的his head resting upon his left forearm，應該是獨立主格結構的{名詞(代詞) +介詞短語}。而其中的resting upon應該是一個<介詞>，為何網上網下均找不到這個><介詞>？