code: function pushBack(t) {
      goog.labs.format.csv.assertToken_(t);
      goog.asserts.assert(goog.isNull(pushBackToken));
      pushBackToken = t;
    }
docs: Special case for terminal comma. 
  Push a single token into the push-back variable.
  @param {goog.labs.format.csv.Token} t Single token.