code: function newPosId (p, f) {
    const pPath = p && p[0]
    const fPath = f && f[0]
    let path
    let uid
    if (pPath && fPath && isSibling(pPath, fPath)) {
      path = [pPath[0], pPath[1]]
      if (f[1].length > p[1].length) {
        const difference = f[1].substring(p[1].length)
        uid = p[1] + halfOf(difference) + cuid()
      } else {
        uid = p[1] + cuid()
      }
    } else {
      path = newPath(pPath, fPath)
      uid = cuid()
    }
  
    return [path, uid]
  }
docs: PosIds and Paths