passing a parrameter read-only ref?

[Martin]

Hi,
i would like to create a function which takes the first parameter 
as a reference to a struct - but assure the calle that the 
reference is read-only. Can this be done?

If i am not mistaken, then the "in" Parameter Storage Class is 
what i want(?). But the documentation states that this feature 
should not be used.

For context: I want to write struct-methods outside the struct 
(like in golang).