code: function(from, to, op) {
      if (op) this.iterN(from - this.first, to - from, op)
      else this.iterN(this.first, this.first + this.size, from)
    }
docs: Iterate over the document. Supports two forms -- with only one argument, it calls that for each line in the document. With three, it iterates over the range given by the first two (with the second being non-inclusive).