parse an XML from a file descriptor and build a tree. NOTE
      that the file descriptor will not be closed when the reader
       is closed or reset.

def readFd(fd, URL, encoding, options):
    """parse an XML from a file descriptor and build a tree. NOTE
      that the file descriptor will not be closed when the reader
       is closed or reset. """
    ret = libxml2mod.xmlReadFd(fd, URL, encoding, options)
    if ret is None:raise treeError('xmlReadFd() failed')
    return xmlDoc(_obj=ret)