Jefferson Airplane's "The Ballad of You and Me and Pooneil"

This morning I listened to Jefferson Airplane's After Bathing at Baxter's.  There's one line in "The Ballad of You and Me and Pooneil" that had always sounded familiar to me, and after listening to it this time, I finally looked it up.  The line in question is "Halfway down the stairs is a stair where I sit and think about you and me."  I finally placed it, but the chronology didn't work out because I knew it from "Halfway Down the Stairs" - a song on The Muppet Show, which was ten years after this album.  But I looked that up and discovered that "Halfway Down the Stairs" is an A.A. Milne poem set to music.  I'm not sure if Jefferson Airplane took their line from Milne's poem or the song it became, but it does seem to have its origin with Milne.