Metadata pointing at a Composition

{
  "resourceType" : "DocumentReference",
  "id" : "example-composition",
  "meta" : {
    "security" : [{
      "system" : "http://terminology.hl7.org/CodeSystem/v3-ActReason",
      "code" : "HTEST"
    }]
  },
  "identifier" : [{
    "system" : "http://healthintersections.com.au/test",
    "value" : "1"
  }],
  "status" : "current",
  "type" : {
    "coding" : [{
      "system" : "http://loinc.org",
      "code" : "11488-4",
      "display" : "Consult note"
    }]
  },
  "category" : [{
    "coding" : [{
      "system" : "http://loinc.org",
      "code" : "LP183761-8",
      "display" : "Report"
    }]
  }],
  "subject" : {
    "reference" : "Patient/xcda",
    "display" : "Henry Levin the 7th"
  },
  "context" : [{
    "reference" : "Encounter/xcda"
  }],
  "event" : [{
    "concept" : {
      "coding" : [{
        "system" : "http://terminology.hl7.org/CodeSystem/v3-ActCode",
        "code" : "HEALTHREC",
        "display" : "health record"
      }]
    }
  },
  {
    "reference" : {
      "reference" : "Observation/example"
    }
  }],
  "period" : {
    "start" : "2010-07-18",
    "end" : "2012-11-12"
  },
  "date" : "2012-01-04T09:10:14Z",
  "author" : [{
    "reference" : "Practitioner/xcda-author",
    "display" : "Harold Hippocrates, MD"
  }],
  "attester" : [{
    "mode" : {
      "coding" : [{
        "system" : "http://hl7.org/fhir/composition-attestation-mode",
        "code" : "professional"
      }]
    },
    "party" : {
      "reference" : "Practitioner/xcda-author",
      "display" : "Harold Hippocrates, MD"
    }
  }],
  "relatesTo" : [{
    "code" : {
      "coding" : [{
        "system" : "http://hl7.org/fhir/document-relationship-type",
        "code" : "replaces"
      }]
    },
    "target" : {
      "reference" : "DocumentReference/old-example"
    }
  }],
  "description" : "Example of a Comprehensive DocumentReference resource. This is fully filled for all mandatory elements and all optional elements.",
  "content" : [{
    "attachment" : {
      "url" : "Composition/example"
    }
  }]
}