state var allpass output

[Martin Czech]

Someone mentioned some time ago, that a state var can not yield
an allpass output.

This is of course wrong. 

Just add notch and bandpass (same polarity) and there you are.
The ap is flat, if the filter is quite ideal.
Any additional poles could prevent notch and bp from canceling,
thus give some ripple.

I did just a simulation to make shure that I tell you no shit.

I works.

m.c.