[tjwii]

I think immutability of arguments should be optional by using a const keyword, like in c:

  int add(const int a, const int b);

[bmm6o]

The downside to this syntactic approach is that the implementation details are leaked in the signature.

[Can_Not]

I think it would better if they were immutable by default and instead of making them const, you would mark them mutable or reference.