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Left-sorted Bongard Problems have the keyword "left-uncertain" on the OEBP.
(Right-sorted Bongard Problems might be called "semi-decidable" in computability lingo, but this is less formal.)
No general algorithm is known for determining whether or not a tile tessellates the plane. However, we are not aware of a proof that such an algorithm is impossible. Even so, we say BP335 ("tessellates the plane vs. does not tessellate the plane") is left-verifiable, since it is, in any case, very difficult to determine some shapes go right.
A more strict (less "fuzzy" right-BP508) definition of "left-verifiable" might require us to be certain there exists no foolproof sorting method, rather than just that the average solver won't come up with one. See keyword "weakhardsort" (left-BP1125) for more on this topic. | 