def label_gen(index):
    """Generates label set for individual gaussian
        index = index of peak location

        output string format:
        'a' + index + "_" + parameter"""
    assert isinstance(index, (float, int))
    pref = str(int(index))
    comb = 'a' + pref + '_'
    cent = 'center'
    sig = 'sigma'
    amp = 'amplitude'
    fract = 'fraction'
    center = comb + cent
    sigma = comb + sig
    amplitude = comb + amp
    fraction = comb + fract
    assert isinstance((center, sigma, amplitude, fraction, comb), str)
    return center, sigma, amplitude, fraction, comb
