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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 modutf7_decode(data: bytes) -> str: """Decode the bytestring using modified UTF-7. Args: data: The encoded bytestring to decode. """

    parts = []
    is_usascii = True
    buf = memoryview(data)
    while buf:
        byte = buf[0]
        if is_usascii:
            if buf[0:2] == b'&-':
                parts.append('&')
                buf = buf[2:]
            elif byte == 0x26:
                is_usascii = False
                buf = buf[1:]
            else:
                parts.append(chr(byte))
                buf = buf[1:]
        else:
            for i, byte in enumerate(buf):
                if byte == 0x2d:
                    to_decode = buf[:i].tobytes()
                    decoded = _modified_b64decode(to_decode)
                    parts.append(decoded)
                    buf = buf[i + 1:]
                    is_usascii = True
                    break
    if not is_usascii:
        to_decode = buf.tobytes()
        decoded = _modified_b64decode(to_decode)
        parts.append(decoded)
    return ''.join(parts)