code: function transformDeleteDelete (a, b, first) {
    // reduce to a normalized case
    if (a.pos > b.pos) {
      return transformDeleteDelete(b, a, !first)
    }
    if (a.pos === b.pos && a.str.length > b.str.length) {
      return transformDeleteDelete(b, a, !first)
    }
    // take out overlapping parts
    if (b.pos < a.pos + a.str.length) {
      var s = b.pos - a.pos
      var s1 = a.str.length - s
      var s2 = s + b.str.length
      a.str = a.str.slice(0, s) + a.str.slice(s2)
      b.str = b.str.slice(s1)
      b.pos -= s
    } else {
      b.pos -= a.str.length
    }
  }
docs: Transform two Deletions --------