code: function(obj) {
      if (obj.normal) { return obj.isParallelTo(this); }
      var theta = this.direction.angleFrom(obj.direction);
      return (Math.abs(theta) <= Sylvester.precision || Math.abs(theta - Math.PI) <= Sylvester.precision);
    }
docs: Returns true if the line is parallel to the argument. Here, 'parallel to' means that the argument's direction is either parallel or antiparallel to the line's own direction. A line is parallel to a plane if the two do not have a unique intersection.