Your definition of EO is significantly different from what I think everyone else assumed it could mean. And, I would like to make the observation that your definition of EO is counter-intuitive - at least to the majority of those who have participated in this thread.

For all intents and purposes, your defined EO is pretty much undefined. Which gets called first - f or g? You're saying that you're not defining this. This is a seriously underdefined execution order, because only bits and pieces (and a very difficult bit and piece to define, as is evidenced by this thread) are defined, but not the entire expression.

It's an interesting definition. I don't think there is much of a gain if only because you've left it pretty much completely undefined. (What if f or g affect $o? What order then?) We've gained so little, and so counter-intuitively, that I'm not sure it's significant enough to put forth code to implement it.

Parallelism is not something we've gained - it's something we still lose. Any definition of execution order is, well, by definition, a definition of serialisation. For example, no matter what, the addition of the two values must happen prior to the assignment to the LHS. We can't add and assign in parallel. Similarly, if we must evaluate all the arguments to functions prior to calling any of them, then we are introducing a sequence point (which is quite invisible, which is why it's counter-intuitive) where we must synchronise everything before paralellising anything.

Further, I'm not sure why you're only defining this for arguments to functions. Why not full subexpressions? That is, defining whether the LHS of a binary operator is evaluated before or after the RHS. Why the arbitrariness of the definition? It would be more intuitive if the whole expression was defined. It's confusing to only go half way, and to stop at some arbitrary point which doesn't seem to have any purpose to it.

You may think I'm opposing you for the sake of opposing. And I'm definitely not as tactful as TimToady. But, at the very least, I hope that I'm helping to refine your idea, or at least the presentation of it. At this moment, however, I do remain unconvinced that there is an idea here that would make life better, so, at the most, I'm hoping that I convince you to see the light. ;-)