In printSuper (common def to process super tree) the result of printing for `Super(This(qual), mix)` tree is `qual.super[mix]`. The behavior of default TreePrinter is to check the symbol of This and print `qual.`)
For typed trees there are cases when we have  tree and `qual` is empty so it's better to check `qual`, otherwise we can have ` .super` as a result code.
I'll merge these cases for common printer.