public default Iterator<Vertex> bothVertices() {
        return this.vertices(Direction.BOTH);
    }
Get both the outgoing and incoming vertices of this edge.
The first vertex in the iterator is the outgoing vertex.
The second vertex in the iterator is the incoming vertex.

@return an iterator of the two vertices of this edge
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/structure/Edge.java