Just a question

I would appreciate very much if you could reveal how you enforce the polynomial type of elements in a polynomial matrix construct, as for example in the following vertical concatenation example:
```

julia> s = Polynomial([0,1],:s)
Polynomial(s)

julia> [s;0]
2-element Array{Polynomial{Int64},1}:
Polynomial(s)
Polynomial(0)
```

I am trying to implement a `RationalTransferFunction` type, where in a vertical concatenation I am getting an array of `Any` (instead an array of `RationalTransferFunction)`, as below:

```
julia> s = rtf('s')
RationalTransferFunction{Int64}(Polynomial(s), Polynomial(1), 0.0)

julia> [s;0]
2-element Array{Any,1}:
RationalTransferFunction{Int64}(Polynomial(s), Polynomial(1), 0.0)
0
```
Many thank in advance for your advice.