Convert from gbk format to big5 representation of chars.

def gbk2big5(chars):
    """
    Convert from gbk format to big5 representation of chars.
    """
    out = ''
    for char in chars:
        if char in _cd.GBK:
            out += _cd.BIG5[_cd.GBK.index(char)]
        else:
            out += char
    return out