There is a game on a tree On each move a proper subtree(i.e. any subtree but not whole tree) has to be clipped off the tree
The player who can't make a move loses.

I am thinking in terms of grundy numbers for each subtree and since each children are independent so xoring all the Gs of them to get the G of root

But this does not seem to work and in the solution they are doing G(root) = XOR(G(1 + child_i)

I don't get it why this +1 is added before xor

Could not find any good explanatory tutorial on this.

Any explanation or redirection to any relevant resources will be appreciated