make ReadHead send + sync #2
Description
if ReadHead is send + sync, then it can be shared between threads. This only makes sense if the relationship between ReadHead and WriteHead can be flipped so that a WriteHead is created from a ReadHead (this is confusing). Or if both are actually from a core object that can create Read and Write heads.
However -- this wont actually be safe because it would then be possible to send the ReadHead multiple times and then generate WriteHeads later and have them fight. MAYBE the core object stores the count of the number of Write heads and makes sure it's only one BUT this is a runtime error and would be better to handle as a compile time error if possible...