from typing import List


def get_qubits_key(list_of_qubits: List[int]) -> str:
    """ from subset of qubit indices get the string that labels this subset
        using convention 'q5q6q12...' etc.
    :param list_of_qubits: labels of qubits

    :return: string label for qubits

     NOTE: this function is "dual" to get_qubit_indices_from_string.
    """

    return 'q' + 'q'.join([str(s) for s in list_of_qubits])