def METHOD_NAME(self, freq, qubit_map=None):  # pragma: no cover
    """Computes the real expectation value of a diagonal observable given the frequencies when measuring in the computational basis.
    Args:
        freq (collections.Counter): the keys are the observed values in binary form
        and the values the corresponding frequencies, that is the number
        of times each measured value/bitstring appears.
        qubit_map (tuple): Mapping between frequencies and qubits. If None, [1,...,len(key)]
    Returns:
        Real number corresponding to the expectation value.
    """
    raise_error(NotImplementedError)