There exists a closed trail that hits each vertex exactly once vs. not so.
Left examples are called "Hamiltonian graphs."
Aaron David Fairbanks
Bongard Problems with "world" any collection of dot-like things that all look the same (any reskinned form of "dots" BP536) vs. other Bongard Problems
See BP538 for possible reskins.
Bongard Problems with "world" any reskinned forms of "dots" (BP536) vs. other Bongard Problems