### Substituting variables in instances of elements of polynomial rings

I am working with

```
R.<q,t> = PolynomialRing(QQ)
```

```
Q = R.fraction_field()
```

```
Sym = SymmetricFunctions(Q)
```

following which I declare `Ht = Sym.Macdonald.Ht()`

. I would now like to evaluate `Ht`

for different relations between `q`

, `t`

, as an example. Or as a simpler example, I'd like to evaluate the `qt_catalan`

function built in Sage with relations like `q = t`

, `q = 1/t`

or `q=t=1`

. How do I go about achieving these? There are several computations that I'd like to maintain in a notebook, so I do not want to declare a quotienting in the base ring itself.