Provenance holding a signature

{
  "resourceType" : "Provenance",
  "id" : "signature",
  "target" : [{
    "reference" : "DocumentReference/example/_history/4"
  }],
  "recorded" : "2015-08-27T08:39:24+10:00",
  "authorization" : [{
    "concept" : {
      "coding" : [{
        "system" : "http://terminology.hl7.org/CodeSystem/v3-ActReason",
        "code" : "TREAT",
        "display" : "treatment"
      }]
    }
  }],
  "activity" : {
    "coding" : [{
      "system" : "http://terminology.hl7.org/CodeSystem/v3-DocumentCompletion",
      "code" : "AU",
      "display" : "authenticated"
    }]
  },
  "agent" : [{
    "type" : {
      "coding" : [{
        "system" : "http://terminology.hl7.org/CodeSystem/contractsignertypecodes",
        "code" : "VERF"
      }]
    },
    "who" : {
      "identifier" : {
        "system" : "urn:ietf:rfc:3986",
        "value" : "mailto://hhd@ssa.gov"
      }
    }
  }],
  "signature" : [{
    "type" : [{
      "system" : "urn:iso-astm:E1762-95:2013",
      "code" : "1.2.840.10065.1.12.1.5",
      "display" : "Verification Signature"
    }],
    "when" : "2015-08-27T08:39:24+10:00",
    "who" : {
      "reference" : "Practitioner/xcda-author"
    },
    "targetFormat" : "application/fhir+xml",
    "sigFormat" : "application/signature+xml",
    "data" : "Li4u"
  }]
}