anchor.diff:          |> exists id rev
           |> task
   
  -    member __.ExistsByExpressionAsync (expr): Task<bool> = 
  -        failwith "Not implemented"
  -
       member __.ExistsBySelectorAsync (dict): Task<bool> = 
           dict 
           |> dictToMap findExpressionToFs
           |> existsBySelector
           <| client
  +        |> task
  +
  +    member __.ExistsByExpressionAsync (exp): Task<bool> = 
  +        exp 
  +        |> ExpressionParser.parse
  +        |> existsBySelector
  +        <| client 
           |> task
   
       member __.ListWithDocsAsync (?options): Task<ListResponse<'doctype>> =
  
positive.code:         member val JsonConverter: JsonConverter = null with get, set
          member val Warning: EventHandler<string> = null with get, set
  
  open Types
  open System.Collections
  
  type Client<'doctype when 'doctype :> CouchDoc>(config: Configuration) = 
      let maybeAddConverter (converter: JsonConverter option) (props: CouchProps) =
  
negative.code:         |> task
  
      member __.CountByExpressionAsync (exp): Task<int> = 
          exp 
          |> ExpressionParser.parse 
          |> countBySelector 
          <| client 
          |> task
  
      member __.CountBySelectorAsync (dict): Task<int> = 
          dict
  