I think I my biology was more rusty than yours, I was confusing inhibitory postsynaptic potentials with "negative action potentials". It looks like there is only one type of action potential coming out of a neuron along an axon, the positive/negative weighting occurs at synaptic junctions carrying over to integration on the dendrite.

That should be fundamentally equivalent to the effect caused by a hypothetical "negative" action potential and a hyper-polarising synapse... and vice versa.

Yes, that was what I was thinking when I (accidently) made up that term.