I think it would be more intuitive to implement an array "appending"
functionality with "concatenating" syntax. The
but is forbidden from use on non-strings. If you want to implement this
preexisting syntax as an array concatenation operator, then it is a blank
slate -- you can decree whatever behaviors you wish for it without
what the combined operator does with strings.
on github there is a comment about adding an array_concat() instead like
js's Array.concat() that would try to avoid the pitfalls of +=, array_push,
and current behaviour of .=
flipping through the mdn for it, its results seem alright.