> The options would then be None/False (=no embedding), True (standard
> embedding), or any indexable.

I implemented the requested modifications, except for the new embeddings.

This is:

- the file sage.rings.universal_cyclotomic_field.all is lazily
imported in sage.rings.all

- the class UniversalCyclotomicField is imported in
sage.rings.universal_cyclotomic_field.all and is the unique point of
entry

- the proposed ways to access the universal cyclotomics is through

sage: UCF.<E> = UniversalCyclotomicField(); UCF
Universal Cyclotomic Field endowed with the Zumbroich basis
sage: E(5)
E(5)
sage: UCF.<zeta> = UniversalCyclotomicField()
sage: zeta(5)
zeta5

UCF.gen(5)
zeta5
UCF.gen(5,2)
zeta5^2

So far, I left open the implementation of the embedding. This is, the
optional argument embedding only takes "True", "False", and "None".
The problem I still have is that internally, the behaviour is always
the same, independent of the embedding. This is to say that

sage: E(5)+E(6)
E(15) + E(15)^4 + E(15)^7 - E(15)^8

should always hold true. And I don't yet see (yet) how to ensure this
- in particular, the embedding for prime n's does at least fix the
embedding of all others. But I don't see if this is enough to ensure
that everything works fine. There is also special behaviour for n
being powers of 2, and I don't see how this influences the embeddings
-- sorry for being ignorant here...

Best, Christian

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