code: function(ln, offset) {
    var s = ln.slice(offset);
    if (!(this.tip.open)) {
      throw({ msg: "Attempted to add line (" + ln + ") to closed container." });
    }
    this.tip.strings.push(s);
  }
docs: Add a line to the block at the tip.  We assume the tip can accept lines -- that check should be done before calling this.