Multiple Subscribers

[hutchig]

We need to discuss the semantics of multiple subscribers.

In order to help flesh this out there is a whiteboard document here:

https://docs.google.com/document/d/1aqtWySO116einKmHX69xtF1Tj8WHY_2MjwPuQqUwkS4/edit?usp=sharing

I decided to share this before populating it but I will be writing in it
over the next couple of days. I am starting with no assumption on the
outcome. Anyone is free to write anything in the document but
please do not destructively edit other people's content apart from things
like spelling and punctuation etc.