def compute_M0_nl(formula, abundance):
    """Compute intensity of the first isotopologue M0.

    Handle element X with specific abundance.

    Parameters
    ----------
    formula : pyteomics.mass.Composition
        Chemical formula, as a dict of the number of atoms for each element:
        {element_name: number_of_atoms, ...}.
    abundance : dict
        Dictionary of abundances of isotopes:
        {"element_name[isotope_number]": relative abundance, ..}.

    Returns
    -------
    float
        Value of M0.

    Notes
    -----
    X represents C with default isotopic abundance.

    """
    M0_intensity = abundance['C[12]'] ** formula['C'] * abundance['X[12]'
        ] ** formula['X'] * abundance['H[1]'] ** formula['H'] * abundance[
        'N[14]'] ** formula['N'] * abundance['O[16]'] ** formula['O'
        ] * abundance['S[32]'] ** formula['S']
    return M0_intensity
