///<summary>A comma-separated list of field names</summary>
internal static FieldCapabilitiesRequestParameters _Fields <T>(this FieldCapabilitiesRequestParameters qs, IEnumerable <Expression <Func <T, object> > > fields) where T : class =>
qs.AddQueryString("fields", fields.Select(e => (Field)e));
///<summary>POST on /{index}/_field_caps <para></para></summary>
///<param name = "index">A comma-separated list of index names; use the special string `_all` or Indices.All to perform the operation on all indices</param>
///<param name = "requestParameters">Request specific configuration such as querystring parameters & request specific connection settings.</param>
public TResponse FieldCapabilities <TResponse>(string index, FieldCapabilitiesRequestParameters requestParameters = null)
where TResponse : class, IOpenSearchResponse, new() => DoRequest <TResponse>(POST, Url($"{index:index}/_field_caps"), null, RequestParams(requestParameters));
///<summary>POST on /{index}/_field_caps <para></para></summary>
///<param name = "index">A comma-separated list of index names; use the special string `_all` or Indices.All to perform the operation on all indices</param>
///<param name = "requestParameters">Request specific configuration such as querystring parameters & request specific connection settings.</param>
public Task <TResponse> FieldCapabilitiesAsync <TResponse>(string index, FieldCapabilitiesRequestParameters requestParameters = null, CancellationToken ctx = default)
where TResponse : class, IOpenSearchResponse, new() => DoRequestAsync <TResponse>(POST, Url($"{index:index}/_field_caps"), ctx, null, RequestParams(requestParameters));
///<summary>POST on /_field_caps <para></para></summary>
///<param name = "requestParameters">Request specific configuration such as querystring parameters & request specific connection settings.</param>
public TResponse FieldCapabilities <TResponse>(FieldCapabilitiesRequestParameters requestParameters = null)
where TResponse : class, IOpenSearchResponse, new() => DoRequest <TResponse>(POST, "_field_caps", null, RequestParams(requestParameters));
