[key]+=val

[Peter Stephenson <p.stephenson@xxxxxxxxxxx>]

I'm intending to make it so that

foo+=([key]+=val)

does something useful (appending to foo[key]) but

foo=([key]+=val)

doesn't (just creates the value), because you're starting with an empty
set of values in foo so there's nothing to append to.  Does this sound
reasonable?  It's a messy layering violation otherwise so it would need
a good reason to do it differently.

pws