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### `uinv_as_power_series`, `ureverse_inv_as_power_series`

`uinv_as_power_series(p,d)`
`ureverse_inv_as_power_series(p,d)`
  :: Computes the truncated inverse as a power series.

return
  univariate polynomial
p
  univariate polynomial
d
  non-negative integer

- For a polynomial p with a non zero constant term, `uinv_as_power_series(p,d)` computes a polynomial r whose degree is at most d such that p*r = 1 mod x^(d+1), where x is the variable of p.
- Let e be the degree of p. `ureverse_inv_as_power_series(p,d)` computes `uinv_as_power_series(p1,d)` for p1=`ureverse(p,e)`.
- The output of `ureverse_inv_as_power_series()` can be used as the input of `rembymul_precomp()`.

    [123] A=(x+1)^5;
    x^5+5*x^4+10*x^3+10*x^2+5*x+1
    [124] uinv_as_power_series(A,5);
    -126*x^5+70*x^4-35*x^3+15*x^2-5*x+1
    [126] A*R;
    -126*x^10-560*x^9-945*x^8-720*x^7-210*x^6+1
    [127] A=x^10+x^9;
    x^10+x^9
    [128] R=ureverse_inv_as_power_series(A,5);
    -x^5+x^4-x^3+x^2-x+1
    [129] ureverse(A)*R;
    -x^6+1

`References`
  section `utrunc`, `udecomp`, `ureverse`, section `udiv`, `urem`, `urembymul`, `urembymul_precomp`, `ugcd`.

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