question: Did a star of M0 and M1 play M2
query: SELECT count(*) WHERE {
  ?x0 ns:film.actor.film/ns:film.performance.character M2 .
  ?x0 ns:film.actor.film/ns:film.performance.film M0 .
  ?x0 ns:film.actor.film/ns:film.performance.film M1
  }