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: SearchParameter</b><a name=\"example-extension\"> </a></p><div style=\"display: inline-block; background-color: #d9e0e7; padding: 6px; margin: 4px; border: 1px solid #8da1b4; border-radius: 5px; line-height: 60%\"><p style=\"margin-bottom: 0px\">Resource SearchParameter &quot;example-extension&quot; </p></div><p><b>url</b>: <code>http://hl7.org/fhir/SearchParameter/example-extension</code></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>: <a href=\"http://hl7.org/fhir\">http://hl7.org/fhir</a></p><p><b>description</b>: Search by url for a participation agreement, which is stored as an extension referencing a DocumentReference</p><p><b>code</b>: part-agree</p><p><b>base</b>: Patient</p><p><b>type</b>: reference</p><p><b>expression</b>: Patient.extension('http://example.org/fhir/StructureDefinition/participation-agreement').value</p><p><b>processingMode</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"
        }
      ]
    }
  ],
  "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"
  ]
}