def weight(self):
        """The total weight of the mobile."""
        #proceed left
        def count_this(branch):
            if type(branch.contents) == type(Weight):
                return branch.contents.weight
            if type(branch.left.contents) != type(Weight):
                return count_this(branch.left)
            else type(branch.right.contents) != type(Weight):
                return count_this(branch.right)


        return count_this(self.branch.left) + count_this(self.branch.right)