When I wrote my comment, I was actually looking at how variables inside a method are declared, not fields (attributes, as they call it in this language) like you mention (seems like the 'variable' keyword is not for local variables, but for attributes).

So I was mainly hoping it is like F#, that forces you to decorate non-immutable variables with the `mutable` keyword, inside a method/function.

However, thinking about it more deeply, this is not so important. Because local variables (in a method) are never shared between threads. So immutability doesn't suffer from them being mutable. Right?