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: SearchParameter</b><a name=\"filter\"> </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;filter&quot; </p></div><p><b>url</b>: <code>http://hl7.org/fhir/SearchParameter/filter</code></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>: 2018-07-26</p><p><b>publisher</b>: Health Level Seven International (FHIR Infrastructure)</p><p><b>contact</b>: FHIR Project: <a href=\"http://hl7.org/fhir\">http://hl7.org/fhir</a></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>: <span title=\" \tthis is the canonical definition for the _id search parameter\t \">Support combination searches when the simple name=value basis of search cannot express what is required</span></p><p><b>code</b>: _filter</p><p><b>base</b>: Resource</p><p><b>type</b>: <span title=\"  all resources  \">special</span></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"
}