Yes, I think the last example is the most annoying one as, to solve it, we have to do a copy which is just something we don't want to do for obvious performance reasons. I have tried to change the parameter type from sequence to openarray with same result. And with an array instead of a sequence, we get the same behavior too. So, changing the semantic for sequences would not be enough, we would have to change the semantic for arrays, too, and kill the whole copy semantic of the language. Not the right way to go, I think.
Maybe a clear distinction between mutable and immutable will indeed solve the issue. The difficulty is to find a balance between the increased complexity of the language and the performance.
