Is the following behaviour intended?

```
prepend!([Sym("A"),Sym("B")],Sym("C")) # returns Vector{SymPy.Sym}: [C,B]
prepend!([Sym("A"),Sym("B")],"C") # returns Vector{SymPy.Sym}: [67,A,B]
```

In the first case, A is replaced by C.

For the second case: Is there a prepend function without conversion to get a Vector{Any}?