pub trait UnitSum {
type Output;
}Expand description
The unit a sum of parts carries, which is the unit each part carries.
A merit adds the halves of a tuple’s contraction together, so a pair whose
halves agree is that unit once. Left unimplemented for a pair whose halves
differ, since adding those names nothing. Implemented concretely rather than
blanketly for the same reason UnitHalves is: a blanket identity would
overlap the pair.