### Units in number fields.

I want to get the expression of a unit in a number field in terms of the base by means of ~~U.log().
~~`U.log()`

.
Here is my ~~code:
~~code:

```
K.<z> = CyclotomicField(21)
U = UnitGroup(K)
U.fundamental_units()
a=(z^3+z^6+z^12)/2+1
```~~[ ~~# a is (3 + i sqrt[7])/4 so it is a unit in ~~K ]
U.log(a)~~K
U.log(a)

I get the error

`ValueError: `~~1/2~~*z^11 **1/2*z^11 - *~~1/2~~z^9 1/2*z^9 + ~~1/2~~*z^8 **1/2*z^8 + *~~1/2~~z^4 1/2*z^4 + 1/2*z + 1/2 is not a ~~unit~~unit

But it is a unit. It is true that

`a = `~~1/2~~*z^11 **1/2*z^11 - *~~1/2~~z^9 1/2*z^9 + ~~1/2~~*z^8 **1/2*z^8 + *~~1/2~~z^4 1/2*z^4 + 1/2*z + ~~1/2.~~1/2

.

What I'm doing ~~wrong~~wrong ?