Molecular composition in dict format
    (ex. Glucose {'C': 6, 'H': 12, 'O': 6}).

def composition(mol):
    """Molecular composition in dict format
    (ex. Glucose {'C': 6, 'H': 12, 'O': 6}).
    """
    mol.require("Valence")
    c = Counter()
    for _, a in mol.atoms_iter():
        c += a.composition()
    return c