class BinaryTree
    function __construct(T, value)
        assert(type(T) == "string", "T must be the name of a class")
        assert(type(value) == T, "Value must be of type T.")
        self.kind = T
        self.value = value
        self.left = nil
        self.right = nil
    end

    -- Alternative constructor to enable kind to be inferred from type of value.
    static function of(value) return new BinaryTree(type(value), value) end

    function getKind()  return self.kind end
    function getValue() return self.value end
    function getLeft()  return self.left end
    function getRight() return self.right end

    function setValue(v)
        assert(type(v) == self.kind, $"Value must be of type {self.kind}")
        self.value = v
    end

    function setLeft(b)
        assert(b instanceof BinaryTree and b:getKind() == self.kind,
        $"Argument must be a BinaryTree of type {self.kind}")
        self.left = b
    end

    function setRight(b)
        assert(b instanceof BinaryTree and b:getKind() == self.kind,
        $"Argument must be a BinaryTree of type {self.kind}")
        self.right = b
    end

    function map(f)
        local tree = BinaryTree.of(f(self.value))
        if self.left  then tree:setLeft(self.left:map(f)) end
        if self.right then tree:setRight(self.right:map(f)) end
        return tree
    end

    function show_top_three()
        return $"({self.left:getValue()}, {self.value}, {self.right:getValue()})"
    end
end

local b = BinaryTree.of(6)
b:setLeft (BinaryTree.of(5))
b:setRight(BinaryTree.of(7))
print(b:show_top_three())

local b2 = b:map(|i| -> i * 10)
print(b2:show_top_three())
b2:setValue("six") -- generates an error because "six" is not a number