Planning: privacy semantics

[LB--]

The privacy of namespace members and class members is determined per-member by a specified or default privacy function. Privacy functions take two parameters - the accessor and the accessee - and return true to permit access or false to deny it. The privacy function for a ember can be determined via expression (e.g. calling a function that returns the privacy function) but the privacy function must be known at compile time, even if it gives its result at runtime.

[LB--]

That's way too general. I probably want more restrictive rules...