There is now a concatenation operator for list-type tags. If foo is a list-type tag, i.e., its value is of the form (A1, A2, ..., An), then a line foo += (B1, B2, ..., Bm) will append the given list to the list currently assigned to foo, i.e., the result in the example will be the same as writing foo (A1, A2, ..., An, B1, B2, ..., Bm). Concatenation works for any list element type, including lists, but it can only be applied to the outermost list level.
The following few changes require programmer attention, as old code will not generate compiler errors, but cease to function properly: