Search parameter for an extension example

{
  "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"]
}