Search parameter for an extension example

{
  "resourceType" : "SearchParameter",
  "id" : "example-extension",
  "text" : {
    "status" : "extensions",
    "div" : "<div xmlns=\"http://www.w3.org/1999/xhtml\"><p><b>Generated Narrative: SearchParameter</b><a name=\"example-extension\"> </a><a name=\"hcexample-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>StructureDefinition Work Group</b>: fhir</p><p><b>url</b>: <code>http://hl7.org/fhir/SearchParameter/example-extension</code></p><p><b>name</b>: ExampleSearchParameterOnAnExtension</p><p><b>title</b>: Example Search Parameter on an extension</p><p><b>status</b>: draft</p><p><b>experimental</b>: true</p><p><b>publisher</b>: HL7 International / FHIR Infrastructure</p><p><b>contact</b>: <a href=\"http://www.hl7.org/Special/committees/fiwg\">http://www.hl7.org/Special/committees/fiwg</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>"
  },
  "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"]
}