{
  "@context": {
    "ex": "http://example.org/vocab#",
    "ex:p1": {
      "@type": "@id"
    }
  },
  "@graph": [
    {
      "@id": "_:a1",
      "ex:p1": "_:a4"
    },
    {
      "@id": "_:a2",
      "ex:p1": "_:a3"
    },
    {
      "@id": "_:a3",
      "ex:p2": "Foo"
    },
    {
      "@id": "_:a4",
      "ex:p2": "Foo"
    }
  ]
}