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Left: the left hand side is enough to communicate the answer; the left pattern can be seen without the counterexamples on the right.
Right: the right hand side is enough to communicate the answer; the right pattern can be seen without counterexamples on the left.
Flipping a BP will switch its sorting.
The following is taken from the comments on page BP513 (keyword left-narrow):
Call a pattern "narrow" if it is likely to be noticed in a collection of examples, without any counterexamples provided.
A collection of triangles will be recognized as such; "triangles" is a narrow pattern. A collection of non-triangular shapes will just be seen as "shapes"; "not triangles" is not narrow.
Narrow patterns tend to be phrased positively ("is [property]"), while non-narrow patterns opposite narrow patterns tend to be phrased negatively ("is not [property]"). | 
