Formal Definition for _filter parameter

{
  "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"
}