def _to_addr(worksheet, row, col, row_fixed=False, col_fixed=False):
    """converts a (0,0) based coordinate to an excel address"""
    addr = ''
    A = ord('A')
    col += 1
    while col > 0:
        addr = chr(A + (col - 1) % 26) + addr
        col = (col - 1) // 26
    prefix = "'%s'!" % worksheet if worksheet else ''
    col_modifier = '$' if col_fixed else ''
    row_modifier = '$' if row_fixed else ''
    return prefix + '%s%s%s%d' % (col_modifier, addr, row_modifier, row + 1)
