def changechr(chr):
    """change between two chr versions
  """
    if chr.isdigit() or chr in ('X', 'Y', 'M'):
        return 'chr' + chr
    elif chr == 'MT':
        return 'chrM'
    elif chr == 'chrM':
        return 'MT'
    elif chr[0:3] == 'chr':
        return chr[3:]
    else:
        return chr
