docs: function(obj) {
      if (obj.anchor) { return obj.distanceFrom(this); }
      var V = obj.elements || obj;
      if (V.length != this.elements.length) { return null; }
      var sum = 0, part;
      this.each(function(x, i) {
        part = x - V[i-1];
        sum += part * part;
      });
      return Math.sqrt(sum);
    }