When I try to define this simple function

```
getCoeff(SeriesData(_, _, coeff_, _, _, _)) := coeff((2))
```

I get an error:

```
SeriesData: Coefficient specification coeff_ in SeriesData(_,_,coeff_,_,_,_) is not a list.
```

Can someone explain what's wrong with that? My understanding was that `SetDelayed`

One should not complain about such things until I actually evaluate an expression that contains `getCoeff`

it should not try to evaluate `Part(coeff,2)`

,

No error is output for the similar function

```
f(g(_, _, x_, _, _, _)) := x((2))
```

and also `getCoeff`

works as expected:

```
getCoeff@Series(E^((Pi) x), {x, 0, 3})
```

outputs $ pi $,

I know the "right" way to manipulate `SeriesData`

I just want to understand this case for its own sake.