nested call variable shadowing

shadowing variable names doenst seem to work correctly.

neither with custom name:
```nim
echo:
  @[@[1,2],@[3,4]].items |>
    map[a](
      a.items |> map[a](a+1).toSeq()).
    toSeq()
```
nor with it:
```nim
echo:
  @[@[1,2],@[3,4]].items |>
    map(
      it.items |> map(it+1).toSeq()).
    toSeq()
```