M. M. Bongard, Pattern Recognition, Spartan Books, 1970, p. 223.
BP1232 is almost exactly the same, but allowing for multiple curves and multiple self-crossings.
Adjacent-numbered pages: BP25 BP26 BP27 BP28 BP29  *  BP31 BP32 BP33 BP34 BP35
nice, finished, traditional, bongard
crosscurve [smaller | same | bigger] zoom in right (curve_or_outline)
Mikhail M. Bongard