Formal Definition for _filter parameter

{
  "resourceType": "SearchParameter",
  "id": "filter",
  "text": {
    "status": "generated",
    "div": "<div xmlns=\"http://www.w3.org/1999/xhtml\"><p><b>Generated Narrative with Details</b></p><p><b>id</b>: filter</p><p><b>url</b>: <b>http://hl7.org/fhir/SearchParameter/filter</b></p><p><b>version</b>: 1</p><p><b>name</b>: FilterSearchParameter</p><p><b>status</b>: draft</p><p><b>experimental</b>: false</p><p><b>date</b>: 26/07/2018</p><p><b>publisher</b>: Health Level Seven International (FHIR Infrastructure)</p><p><b>contact</b>: </p><p><b>description</b>: This is the formal declaration for the _filter parameter, documented at [http://hl7.org/fhir/search_filter.html](http://hl7.org/fhir/search_filter.html)</p><p><b>purpose</b>: Support combination searches when the simple name=value basis of search cannot express what is required</p><p><b>code</b>: _filter</p><p><b>base</b>: Resource</p><p><b>type</b>: special</p></div>"
  },
  "url": "http://hl7.org/fhir/SearchParameter/filter",
  "version": "1",
  "name": "FilterSearchParameter",
  "status": "draft",
  "experimental": false,
  "date": "2018-07-26",
  "publisher": "Health Level Seven International (FHIR Infrastructure)",
  "contact": [
    {
      "name": "FHIR Project",
      "telecom": [
        {
          "system": "url",
          "value": "http://hl7.org/fhir"
        }
      ]
    }
  ],
  "description": "This is the formal declaration for the _filter parameter, documented at [http://hl7.org/fhir/search_filter.html](http://hl7.org/fhir/search_filter.html)",
  "purpose": "Support combination searches when the simple name=value basis of search cannot express what is required",
  "code": "_filter",
  "base": [
    "Resource"
  ],
  "type": "special"

  "resourceType" : "SearchParameter",
  "id" : "filter",
  "extension" : [{
    "url" : "http://hl7.org/fhir/StructureDefinition/structuredefinition-wg",
    "valueCode" : "fhir"
  }],
  "url" : "http://hl7.org/fhir/SearchParameter/filter",
  "version" : "1",
  "name" : "FilterSearchParameter",
  "title" : "Filter Search Parameter",
  "status" : "draft",
  "experimental" : false,
  "date" : "2018-07-26",
  "publisher" : "HL7 International / FHIR Infrastructure",
  "contact" : [{
    "telecom" : [{
      "system" : "url",
      "value" : "http://www.hl7.org/Special/committees/fiwg"
    }]
  }],
  "description" : "This is the formal declaration for the _filter parameter, documented at [http://hl7.org/fhir/search_filter.html](http://hl7.org/fhir/search_filter.html)",
  "purpose" : "Support combination searches when the simple name=value basis of search cannot express what is required",
  "code" : "_filter",
  "base" : ["Resource"],
  "type" : "special"

}