### Creating a polynomial from a string with symbolic constants

Below is some code that does the following. First, define `R`

to be the univariate polynomial ring `Q_2[x]`

, and let `f(x)`

be the polynomial `x^2 - 2`

in this ring. Then let `K`

be the totally ramified quadratic extension defined by this polynomial, and call its generator a. Let `S`

be the polynomial ring `K[y]`

. Then I would like to define a polynomial `y + a`

in `S`

. This works find if I write `f1 = S(y+a)`

, but it fails if I try `f2 = S('y + a')`

. However, I need to be able to define my polynomial from a string as in the `f2`

case. Can anyone help?

```
R.<x> = Qp(2,100)[]
f = R(x^2 - 2)
K.<a> = Qp(2,100).ext(f)
S.<y> = K[]
# The next line works
f1 = S(y + a)
# The next one throws an error
f2 = S('y + a')
```