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Left-sorted Problems have the keyword "left-listable" on the OEBP.
All the possible left examples for the BPs on the left side of this problem could be listed in one infinite sequence. Right examples here are Problems for which no such sequence can exist.
This depends on deciding what images should be considered "the same thing", which is subjective and context-dependent.
All examples in this Bongard Problem have an infinite left side (they do not have the keyword left-finite).
The mathematical term for a set that can be organized into an infinite list is a "countably infinite" set, as opposed to an "uncountably infinite" set.
Another related idea is a "recursively enumerable" a.k.a. "semi-decidable" set, which is a set that a computer program could list the members of.
The keyword "left-listable" is meant to be for the more general idea of a countable set, which does not have to do with computer algorithms.
Note that this is not just BP940 (right-listable) flipped.
It seems in practice, Bongard Problems that are left-listable are usually also right-listable because the whole class of relevant examples is listable. A keyword for just plain "listable" may be more useful. Or instead keywords for left- versus right- semidecidability, in the sense of computing. - Aaron David Fairbanks, Jan 10 2023 | 
