> Couldn't we just make "expr[expr] = expr" and "variable = expr" the grammar rules, and code everything in there?

Well, we could, but it'd require significant changes to the parser. Consider (foo)=bar. At the point where the parser sees the =, the bytecode for foo has already been written. It might be possible to delete that bytecode, write the bytecode for the second expression, and then add the write-to-variable bytecode. But that's gross. So, we'd have to switch to a proper AST first.

Also, += and friends would have to change to read+modify+write bytecodes to avoid having 78 bytecodes for this (13 operators * 6 destination types). Or we'd need to change the bytecodes to encode the destination in the bytecode, instead of always pushing it onto the stack.

And I've got another idea: If we impose the rule that all functions with the same name have the same type - reference or no reference - of parameters and return values, the parser would know whether a certain function parameter needs a reference. That way, we wouldn't need special proplist references, because no reference would be created that the scripter didn't intend.