The rest of the program is given in the link above. I don't see a way to delete that.
I tried doing this instead:
vertices(pre_order($root))
but that just gives the output of pre_order($root) again and says the number of vertices is 0.

In reply to Re^2: program that counts all the vertices in a given tree by o_0
in thread program that counts all the vertices in a given tree by o_0

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