This Tree is far from complete and needs lots of functionality. For instance, we may need to check whether the Tree contains a specific value. To be able to do this, we need to add the following method to our Tree:

static func contains(_ key: Element, tree: Tree<Element>) -> Bool { 
    switch tree { 
    case .leaf(let element): 
        return key == element 
    case node(let lhs, let rhs): 
        return contains(key, tree: lhs) || contains(key, tree: rhs) 
    } 
} 

We can test the contains method as follows:

let isFound = Tree.contains("First", tree: functionalTree)  
print(isFound) // prints true