`dom` should not require a type annotation for the return type
For almost all uses, the return type of
dom should be inferrable from the source type, so we should change the setup appropriately.
The only exception is taking, say, the
dom coPset of a
gmap — but that's only used once in Iris (or clients, according to @jung), and clients can convert the resulting
gset to a
coPset if they want.