def chemical_symbol_first(chaine):
    """ Makes sure that the element symbol comes before the number.
    This is only to comply with the xml format that prohibits string starting
    with a number.
    """
    valeur_num = ''
    valeur_car = ''
    for truc in range(len(chaine)):
        if chaine[truc] in '0123456789':
            valeur_num += chaine[truc]
        else:
            valeur_car += chaine[truc]
    return valeur_car + valeur_num
