code: function(callback, a, b) {
         return callback.length == 1 ? callback(b) : callback(a, b)
       }
docs: Call an iterator callback with arguments a, or b
  depending on the arity of the callback.
  
  @param  {function} callback
  @param  {mixed} a
  @param  {mixed} b
  @return {mixed}
  @api private