code: function(callback) {
      this._cacheBackboneTraces();
      for (var i=0; i < this._cachedTraces.length; ++i) {
        callback(this._cachedTraces[i]);
      }
    }
docs: invokes a callback for each connected stretch of amino acids. these stretches are used for all trace-based rendering styles, e.g. sline, line_trace, tube, cartoon etc.