'Give the string expression of a unit in term of the basis.
Units are displayed by decreasing power.'

def print_unit_base(self, unit):

    res = S.One
    factor = unit.scale_factor
    vec = self._system.dim_vector(unit.dimension)
    for (u, p) in sorted(zip(self._base_units, vec), key=(lambda x: x[1]), reverse=True):
        factor /= (u.scale_factor ** p)
        if (p == 0):
            continue
        elif (p == 1):
            res *= u
        else:
            res *= (u ** p)
    return (factor * res)
