Vertices that can reach the given vertices (BFS order)
VertexComponent[g, {v1, v2, …}]
VertexComponent[g, {v1, v2, …}, k]
VertexComponent[g, {v1, v2, …}, {k}]
VertexComponent[{v -> w, …}, …]
VertexComponent[{1 -> 2, 1 -> 5, 2 -> 3, 3 -> 4, 4 -> 5, 6 -> 3, 4 -> 6}, {2}]
→ {2, 1}