def unknown_operation(num_qubits: int) -> 'SymbolInfo':
        """Generates a SymbolInfo object for an unknown operation.

        Args:
            num_qubits: the number of qubits in the operation
        """
        symbol_info = SymbolInfo([], [])
        for _ in range(num_qubits):
            symbol_info.colors.append('gray')
            symbol_info.labels.append('?')
        return symbol_info