<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:

def mkdir(self, foldername):
        'Create a new subfolder and return the new JFSFolder'
        #url = '%s?mkDir=true' % posixpath.join(self.path, foldername)
        url = posixpath.join(self.path, foldername)
        params = {'mkDir':'true'}
        r = self.jfs.post(url, params)
        self.sync()
        return r