the whole point of this discipline is that this dot is simply a "generic Operator" you can use whatever symbol you want all you really know - is the axiom you

the whole point of this discipline is that this dot is simply a "generic Operator" you can use whatever symbol you want all you really know - is the axiom you're given, which transforms one tree structure of operator application into a different one NAND suggested in replies isn't THE operator, it's AN operator that follows given rules