def compute_M1_nl(formula, abundance):
    """Compute intensity of the second isotopologue M1.

    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 M1.

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

    """
    M1_intensity = formula['C'] * abundance['C[12]'] ** (formula['C'] - 1
        ) * abundance['C[13]'] * 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'] + formula['X'] * abundance['C[12]'] ** formula['C'
        ] * abundance['X[12]'] ** (formula['X'] - 1) * abundance['X[13]'
        ] * abundance['H[1]'] ** formula['H'] * abundance['N[14]'] ** formula[
        'N'] * abundance['O[16]'] ** formula['O'] * abundance['S[32]'
        ] ** formula['S'] + formula['H'] * abundance['C[12]'] ** formula['C'
        ] * abundance['X[12]'] ** formula['X'] * abundance['H[1]'] ** (formula
        ['H'] - 1) * abundance['H[2]'] * abundance['N[14]'] ** formula['N'
        ] * abundance['O[16]'] ** formula['O'] * abundance['S[32]'] ** formula[
        'S'] + formula['N'] * abundance['C[12]'] ** formula['C'] * abundance[
        'X[12]'] ** formula['X'] * abundance['H[1]'] ** formula['H'
        ] * abundance['N[14]'] ** (formula['N'] - 1) * abundance['N[15]'
        ] * abundance['O[16]'] ** formula['O'] * abundance['S[32]'] ** formula[
        'S'] + formula['O'] * 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'] - 1) * abundance['O[17]'] * abundance['S[32]'] ** formula['S'
        ] + formula['S'] * 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'] - 1) * abundance['S[33]']
    return M1_intensity
