Prevent 'add_child' from overwriting the 'child' (raise error)

[nrathaus]

I think it is a good idea to change the code to this:

    def add_child(self, child):
        """
        Sets the parameter as the child of the node

        :return: the child node
        """
        if self.child is not None:
            raise ValueError("Child is being overwritten")

        self.child = child
        return self.child

Or similar, to prevent repeated add_child being called when 'self.child' is already set, thus losing the previous one

[tomato42]

why?
it wouldn't protect against setting the child directly, and in some complex decision graphs, it may be easier to first create a simple structure and then modify it in place...