@Override
    public String toString() {
        return StringFactory.graphComputerString(this);
    }
An {@code ExecutorService} that schedules up background work. Since a {@link GraphComputer} is only used once
for a {@link VertexProgram} a single threaded executor is sufficient.
tinkergraph-gremlin/src/main/java/org/apache/tinkerpop/gremlin/tinkergraph/process/computer/TinkerGraphComputer.java