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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 _unescape(v):
 """Unescape characters in a TOML string."""

    i = 0
    backslash = False
    while i < len(v):
        if backslash:
            backslash = False
            if v[i] in _escapes:
                v = v[:i - 1] + _escape_to_escapedchars[v[i]] + v[i + 1:]
            elif v[i] == '\\':
                v = v[:i - 1] + v[i:]
            elif v[i] == 'u' or v[i] == 'U':
                i += 1
            else:
                raise TomlDecodeError("Reserved escape sequence used")
            continue
        elif v[i] == '\\':
            backslash = True
        i += 1
    return v