An identity $E$ that obeys all the claimed properties is $$ E: x+(y+z) = (x+y)+w \hbox{ for all } x,y,z,w.$$ - $E$ is implied by triple constancy (and hence by constancy): obvious since both sides are constant in this case - $E$ does not imply triple constancy (and hence does not imply constancy either): follows from considering the [left-zero semigroups][1] $x+y=x$ mentioned by arsmath - $E$ implies associativity: obvious by specializing to $w = z$ - $E$ is not implied by associativity: follows from considering (say) addition on the integers This candidate $E$ was located by pursuing the analysis in Pace's answer to isolate the form that $E$ had to take as much as possible, as described in the comments to that answer. With a little more effort, it should be possible to entirely classify (up to relabelings and symmetries) the full set of identities $E$ that answer the question. Here is the [Hasse diagram][2] of the various identities discussed on this page, where the ordering is from stronger identities to weaker ones: [![Hasse diagram][3]][3] It might be a suitable undergraduate research project to extend this diagram to cover other short identities for magmas. EDIT: It might be a suitable graduate research project to find a way to do this automatically using proof assistants and possibly also machine learning/AI tools. [1]: https://en.wikipedia.org/wiki/Null_semigroup#Left_zero_semigroup [2]: https://en.wikipedia.org/wiki/Hasse_diagram [3]: https://i.sstatic.net/3ewey.png