### subgroup of number field unit group

I would like to be able to create subgroups of a unit group generated by given elements. I haven't, thus far, been able to find a way to make this work. The following doesn't work:

```
N.<a> = NumberField(x^3+2)
G = N.unit_group()
g = G.random_element()
G.subgroup([g])
```

Can anyone help me find a ~~workaround to do what I want to do? I would like to be able to create subgroups of a unit group generated by certain elements. I haven't, thus far, been able to find a way to make this work.~~workaround?