Search parameter for an extension example

{
  "resourceType": "SearchParameter",
  "id": "example-extension",
  "text": {
    "status": "generated",
    "div": "<div xmlns=\"http://www.w3.org/1999/xhtml\"><p><b>Generated Narrative with Details</b></p><p><b>id</b>: example-extension</p><p><b>url</b>: <a>http://hl7.org/fhir/SearchParameter/example-extension</a></p><p><b>name</b>: Example Search Parameter on an extension</p><p><b>status</b>: draft</p><p><b>experimental</b>: true</p><p><b>publisher</b>: Health Level Seven International (FHIR Infrastructure)</p><p><b>contact</b>: </p><p><b>code</b>: part-agree</p><p><b>base</b>: Patient</p><p><b>type</b>: reference</p><p><b>description</b>: Search by url for a participation agreement, which is stored in a DocumentReference</p><p><b>expression</b>: DocumentReference.extension('http://example.org/fhir/StructureDefinition/participation-agreement')</p><p><b>xpath</b>: f:DocumentReference/f:extension[@url='http://example.org/fhir/StructureDefinition/participation-agreement']</p><p><b>xpathUsage</b>: normal</p><p><b>target</b>: DocumentReference</p></div>"
  },
  "url": "http://hl7.org/fhir/SearchParameter/example-extension",
  "name": "Example Search Parameter on an extension",
  "status": "draft",
  "experimental": true,
  "publisher": "Health Level Seven International (FHIR Infrastructure)",
  "contact": [
    {
      "telecom": [
        {
          "system": "url",
          "value": "http://hl7.org/fhir"
        }
      ]
    }
  ],
  "code": "part-agree",
  "base": [
    "Patient"
  ],
  "type": "reference",
  "description": "Search by url for a participation agreement, which is stored in a DocumentReference",
  "expression": "DocumentReference.extension('http://example.org/fhir/StructureDefinition/participation-agreement')",
  "xpath": "f:DocumentReference/f:extension[@url='http://example.org/fhir/StructureDefinition/participation-agreement']",
  "xpathUsage": "normal",
  "target": [
    "DocumentReference"
  ]

  "resourceType" : "SearchParameter",
  "id" : "example-extension",
  "extension" : [{
    "url" : "http://hl7.org/fhir/StructureDefinition/structuredefinition-wg",
    "valueCode" : "fhir"
  }],
  "url" : "http://hl7.org/fhir/SearchParameter/example-extension",
  "name" : "ExampleSearchParameterOnAnExtension",
  "title" : "Example Search Parameter on an extension",
  "status" : "draft",
  "experimental" : true,
  "publisher" : "HL7 International / FHIR Infrastructure",
  "contact" : [{
    "telecom" : [{
      "system" : "url",
      "value" : "http://www.hl7.org/Special/committees/fiwg"
    }]
  }],
  "description" : "Search by url for a participation agreement, which is stored as an extension referencing a DocumentReference",
  "code" : "part-agree",
  "base" : ["Patient"],
  "type" : "reference",
  "expression" : "Patient.extension('http://example.org/fhir/StructureDefinition/participation-agreement').value",
  "processingMode" : "normal",
  "target" : ["DocumentReference"]

}