whole_func_string: function _normalizeTask(task) {
              var _task = task;
              if (task.length < 3) {
                  _task = function _task(err, res, next) {
                      // wrap task with no callback
                      var result = task.call(null, err, res);
                      next.call(null, null, result);
                  };
              } else if (task.length === 3) {
  
                  // normal
                  _task = task;
  
              } else if (task.length === 4) {
  
                  // with index
                  _task = function _task(err, res, next, index) {
                      task.call(null, err, res, next, index);
                  };
  
              } else {
                  throw new Error('args length invalid');
              }
              return _task;
          }
func_code_string: function _normalizeTask(task) {
              var _task = task;
              if (task.length < 3) {
                  _task = function _task(err, res, next) {
                      // wrap task with no callback
                      var result = task.call(null, err, res);
                      next.call(null, null, result);
                  };
              } else if (task.length === 3) {
  
                  // normal
                  _task = task;
  
              } else if (task.length === 4) {
  
                  // with index
                  _task = function _task(err, res, next, index) {
                      task.call(null, err, res, next, index);
                  };
  
              } else {
                  throw new Error('args length invalid');
              }
              return _task;
          }
func_documentation_string: taskの引数をラップして共通化します。
  @memberof Bucks
  @instance
  @private
  @param {Function} task ex) task(err, res, next) | task(err, res) | task(err) | task()
  @throws {Error} args length invalid
  @return {functio} task(err, res, next)