`+=` is great for commuting addition operations, but not for non-commuting operations.  The order of list-assembly is important.  `+=` appends, and the root needs to be added at the front.  But more generally, `x += [y]` conceals the operation order, and sometimes people forget.  `x = x + [y]` and `x = [y] + x` are  clearer for non-commuting operations.