Digraphoid

We now wish to consider what part of the theory of directed graphs can be built up in the wider context of graphoids/matroids. To this end, we shall introduce the concepts of digraphoid (short for “directed graphoid”) and orientable graphoid. […] A digraphoid is a structure consisting of: (1º) a graphoid, and (2º) a partitioning of each circuit and cocircuit of the graphoid, each being partitioned into two sets; this partitioning is to satisfy the axiom: […]