smichr/sympy — comment on commit a00f5e8c5ffec22f2a15acc2b105478cca9a6619
File:  line 0
By asmeurer:

I think it would need to be a hint, because it's actually a different kind of substitution than what we do now. In other words, something like

```
(x + y).subs([(x, y), (y, z)])
```

will return something different if you do it sequentially than if you do it all at once, and actually, you would potentially want both behaviors depending on what you are doing. There was a discussion on the mailing list about this a while back. There it was suggested that to do it all at once you could use dummy variables, but actually, it's more efficient to literally do it all at once, which would require a rewrite of the subs mechanism.