What is ChainMap going to do? Normally, the left-most argument to ChainMap is the "top level" dict, but in a regular union scenario, last value wins. 

Seems like layering the right hand side's dict on top of the left hand side's would match dict union semantics best, but it feels... wrong, given ChainMap's normal left-to-right precedence. And top-mostness affects which dict receives all writes, so if  chain1 |= chain2 operates with dict-like precedence (chain2 layers over chain1), then that also means the target of writes/deletions/etc. changes to what was on top in chain2.