# symbolic substitution is done only partially

I'm trying to use symbolic ring as a media for conversion between complex algebraic entities. However, I've encountered an issue when the substitution does not quite work. The following code illustrates the issue, where only part of the variables are substituted. Namely, symbolic variables `x`

and `t`

remain intact despite of the request to substitute them with `xx`

and `tt`

:

```
R = PolynomialRing(QQ,2,'s')
s = R.gens()
K.<x,t> = R[[]]
f = (s[0]+s[1])*x + (s[0]^2+s[1]^2)*x^2 + t*x^3
fs = SR( f.polynomial() )
print(f'fs: {fs.parent()} {fs.variables()}')
var('ss0','ss1','xx','tt')
fn = fs.subs({SR(s[0]):ss0, SR(s[1]):ss1, SR(t):tt, SR(x):xx})
print(f'fn: {fn.parent()} {fn.variables()}')
```

The output is

```
fs: Symbolic Ring (s0, s1, t, x)
fn: Symbolic Ring (ss0, ss1, t, x)
```

What's wrong?

**UPDATE.**
Apparently, the substitution

```
fn = fs.subs({SR(s[0]):ss0, SR(s[1]):ss1, SR(t.polynomial()):tt, SR(x.polynomial()):xx})
```

works as expected. So, it seems that Sage does not recognize `SR(t)`

and `SR(t.polynomial())`

as the same variable. Is this expected?