anchor.diff:  
   /// Enumeration of possible requests to Starling.
   type Request =
  +    /// List all available requests.
  +    | List
       /// Run the language frontend only, with the given request.
       | Frontend of Lang.Frontend.Request
       /// Stop at graph optimisation.
  
positive.code: /// Pretty-prints a response.
  let printResponse mview =
      function
      | Frontend f -> Lang.Frontend.printResponse mview f
      | GraphOptimise g ->
          printUVModelView printGraph mview g
  
negative.code: 
      let starlingR =
          match (requestFromStage opts.stage) with
          // Handle pseudo-requests here, as it's cleaner than doing so in
          // runStarling.
          | Some Request.List ->
              ok (Response.List (Map.map (fun _ -> fst) requestMap))
          | Some otype -> runStarling times optS backendS verbose otype opts.input
          | None -> fail Error.BadStage
  
  