Replying to ncohen:

Could you also add to your docstring a reference toward a textbook that defines this notion?

Duh. Counting Finite Lattices by Heitzig and Reinhold defines it "- - contains an element which is neither the greatest not the least element of L but comparable to every element of L." On the other hand, On the number of distributive lattices by Erné and (same) Heitzig and Reinhold says "- - if it is either a singleton or the vertical sum of two nonempty posets - -", and vertical sum on two two-element lattice by their definition is the two-element lattice.

I select tscrim as another random victim. Travis, should we define the two-element lattice to be vertically decomposable or indecomposable?

(Or raise `OtherError("developers don't know how to define this")`

? `:=)`

)