Classification of doubly distributive skew hyperfields and stringent hypergroups

A hypergroup is stringent if $a boxplus b$ is a singleton whenever $a eq -b$. A hyperfield is stringent if the underlying additive hypergroup is. Every doubly distributive skew hyperfield is stringent, but not vice versa. We present a classification of stringent hypergroups, from which a classification of doubly distributive skew hyperfields follows. It follows from our classification that every such hyperfield is a quotient of a skew field.