'Process an XHDR command (optional server extension).  Arguments:
- hdr: the header type (e.g. \'subject\')
- str: an article nr, a message id, or a range nr1-nr2
Returns:
- resp: server response if successful
- list: list of (nr, value) strings'

def xhdr(self, hdr, str, file=None):

    pat = re.compile('^([0-9]+)    ?(.*)\n?')
    (resp, lines) = self.longcmd(((('XHDR    ' + hdr) + '    ') + str), file)
    for i in range(len(lines)):
        line = lines[i]
        m = pat.match(line)
        if m:
            lines[i] = m.group(1, 2)
    return (resp, lines)
