[lanna]

It looks like transform(tree.left) returns an Option[Tree] already (otherwise the code would not type check) so the entire if-else in the original code seems redundant and could be replaced with:

    val transformedLeft = transform(tree.left)

[IvanVergiliev]

Just responded to the parent comment as well - there's an additional mutable argument to the real `transform` method so it's unsafe to invoke it directly without first checking if the tree is convertible.