low-level power series interface

[Bill Allombert <allomber@math.u-bordeaux.fr>]

Dear PARI developpers,

There are now a number of low-level PARI functions related to
power series. We should add a code for them:

I suggest to consider this as an alternative to Q (RgXQ: quotient ring).
For example we could pick 'n' (to mean "mod X^n"):

RgX_mullow(P,Q,n)      -> RgXn_mul(P,Q,n)
RgX_modXn_shallow(P,n) -> RgXn_red(P,n)
RgX_modXn_eval(P,Q,n)  -> RgX_RgXn(P,Q,n)

Better suggestion ?

Cheers,
Bill.