  It is rigorously proved that ideal memcapacitors and meminductors are not
passive or lossless devices, nor are they satisfying the weaker notion of
cyclo-passivity, which arises when dropping the requirement of non-negativity
of the storage function. Equivalently, this implies that there exist excitation
profiles that allow to extract more energy from the device than it is supplied
with; so that their energy conversion efficiency exceeds 100%. This means that
ideal memcapacitors and meminductors constitute so-called overunity systems. An
illustrative mechanical analogue is provided that explicitly confirms this
property. Hence, the question arises if ideal memcapacitors and meminductors
will just remain some mathematical toys or artefacts.
