setOutgoing method
Points vertex at halfEdge as its way into the mesh.
What an operation calls after rewiring a loop out from under a vertex. validate asks that the half-edge starts there; what it cannot ask is that the half-edge is still on a live loop, so an operation that moves one says where the vertex goes instead.
Implementation
void setOutgoing(int vertex, int halfEdge) {
_wrote = true;
_outgoing.write(vertex, halfEdge);
}