title: More complex joins with mql
body: <p>I tried to find actors sharing the same first movie. 
  It was easy to find first movies of all actors. </p>
  
  <pre><code>[{
      "type": "/film/actor",
      "name": null,
      "/film/actor/film": [{
        "film": [{
          "name":          null,
          "initial_release_date": null
        }],
        "sort":  "film.initial_release_date",
        "limit": 1
      }]
  }]
  </code></pre>
  
  <p>After that I wanted to make a kind of intersection of results of two queries. As a start,  I noticed an example in the query editor at freebase.com where we need two actors participating in the same movie </p>
  
  <pre><code>[{
    "a:starring": [{
      "actor": "Joe Pesci"
    }],
    "b:starring": [{
      "actor": "Robert de Niro"
    }],
    "name": null,
    "id":   null,
    "starring": [{
      "actor": null
    }],
    "type": "/film/film"
  }]
  </code></pre>
  
  <p>But when I tried to make similar changes prefixing my two identical queries with something like "a:film", "b:film" there was always error. Probably I'm too used to SQL and think in terms of temporal sub-queries and I should think some other way.  </p>