I'm not sure what you find confusing: can you give a short example. I think I'm fine with both.
you must keep in mind that the result is stored in the first argument, even though concatenation can easily be visualized.
this, with a little sloppyness, can easily lead to a refactoring that transposes the arguments and ends up in something wrong.
but i don't mean it as an argument against their inclusion in Alexandria. one can shoot their legs in many other ways... :)