Formal Definition for _filter parameter

{
  "resourceType" : "SearchParameter",
  "id" : "filter",
  "text" : {
    "status" : "extensions",
    "div" : "<div xmlns=\"http://www.w3.org/1999/xhtml\"><p><b>Generated Narrative: SearchParameter</b><a name=\"filter\"> </a><a name=\"hcfilter\"> </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>StructureDefinition Work Group</b>: fhir</p><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>title</b>: Filter Search Parameter</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>: HL7 International / FHIR Infrastructure</p><p><b>contact</b>: FHIR Project: <a href=\"http://www.hl7.org/Special/committees/fiwg\">http://www.hl7.org/Special/committees/fiwg</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>: 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>"
  },
  "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" : [{
    "name" : "FHIR Project",
    "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"
}