Chemical formula.
    Atoms should be arranged in order of C, H and other atoms.
    Molecules should be arranged in order of length of formula text.

def formula(mol):
    """Chemical formula.
    Atoms should be arranged in order of C, H and other atoms.
    Molecules should be arranged in order of length of formula text.
    """
    mol.require("Valence")
    mol.require("Topology")
    total_cntr = Counter()
    for m in sorted(mols_iter(mol), key=len, reverse=True):
        cntr = Counter()
        for i in m:
            cntr += mol.atom(i).composition()
        text = []
        Cs = cntr.pop("C", 0)
        if Cs:
            text.append("C")
            if Cs > 1:
                text.append(str(Cs))
        Hs = cntr.pop("H", 0)
        if Hs:
            text.append("H")
            if Hs > 1:
                text.append(str(Hs))
        heteros = sorted(cntr.items(), key=lambda x: atom_number(x[0]))
        for k, v in heteros:
            text.append(k)
            if v > 1:
                text.append(str(v))
        total_cntr["".join(text)] += 1
    total = sorted(total_cntr.items(), key=lambda x: len(x[0]), reverse=True)
    total_text = []
    for k, v in total:
        if v > 1:
            total_text.append(str(v) + k)
        else:
            total_text.append(k)
    return ".".join(total_text)