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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _unicode_sub_super(string, mapping, max_len=None):
 """Try to render a subscript or superscript string in unicode, fall back on ascii if this is not possible"""

    string = str(string)
    if string.startswith('(') and string.endswith(')'):
        len_string = len(string) - 2
    else:
        len_string = len(string)
    if max_len is not None:
        if len_string > max_len:
            raise KeyError("max_len exceeded")
    unicode_letters = []
    for letter in string:
        unicode_letters.append(mapping[letter])
    return ''.join(unicode_letters)