Over on the APL Orchard, I previously commented, "Nice. Prepending an empty vector lets us control the prototype of a potentially empty vector." However, Adám cautioned, "Relying on that seems dangerous. Better prepend a cell of the type you want, and then dropping (sic) it."
My takeaway from the brief discussion was that concatenation—and perhaps primitives more generally—only have a implementation-defined semantics when operating on two empty arguments, but I suspect this characterization is a bit over-eager.
What parts of primitive operations on empty arrays are considered stable and which parts are considered Wild West™?
Cheers in advance.