In reply to:

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I think you have to define x_2 explicitly .. My brain can't comprehend why,

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Yeah, I don't understand it either, as the original equation defines x_2 correctly. Perhaps there's a hidden divide-by-zero in the derivation (a bit like the one in the standard "proof" that 1=2).

Anyway, back to my DSSSL hacking - I've almost got something that interprets a user tag "print" to decide whether or not to output each subtree.