struct Tree {
mut:
	garbage int
	parent  ?&Tree
}

fn main() {
	// `parent` is not declared as mutable!
	parent := Tree{
		garbage: 11
	}
	// taking a reference of `parent` and putting it under a `mut:` struct field
	mut child := Tree{
		parent: &parent
	}

	// unwrap the reference of `parent`, but declare it as `mut`
	mut inner_parent := child.parent?
	inner_parent.garbage = 777 // !we can mutate `parent` freely!
	println(parent.garbage) // 777
}