A shelf that has an opposite resonance at the corner frequency - i.e a slight dip with a shelving boost and a peak with a shelving cut. ... I forgot to continue with the Gerzon shelf discussion. It looks like he never presented an analog circuit for that.
I've been extensively modding the EQ in my Wheatstone console and trying to get that type of shelf from a state variable circuit. I got a shelf with adjustable resonance, where it has a peak at the top of the slope and a dip at the bottom of the slope. Still working on a shelf with only the dip. It may involve combining the BP and HP outputs. I know it can be done, because the Undertone Audio console does it with a potentiometer that fades between bell and shelf.