anchor.diff:                 headed "Command" (cmdprint |> Seq.singleton)
                ]  
   
  -
  -    // let printQuery (zterm : Backends.Z3.Types.ZTerm) : Doc = 
  -    //    vsep  
  -    //        [ headed "Variables" <|
  -    //              List.map printMarkedVar (findVarsGrass zterm)
  -    //          headed "Grasshopper terms" <| 
  -    //              [ printTerm
  -    //                  (Core.Expr.Pretty.printBoolExpr (printSym printMarkedVar)) 
  -    //                  printExprGrass 
  -    //                  printExprGrass 
  -    //                  zterm.SymBool ]
  -    //        ]
  -
       let printGrassError e = 
           failwith "not implemented yet" 
   
  
positive.code:     open Starling.Core.Model.Pretty
      open Starling.Core.Symbolic.Pretty
  
      let printMarkedVar (v : MarkedVar) : Doc =
          match v with 
          | Before s -> String "before_" <-> String s 
  
negative.code:         | Bool b -> printBoolExpr pVar b
          | _ -> failwith "Unimplemented for Grasshopper backend." 
  
      /// Pretty-prints a symbolic sentence 
      let rec printSym (pReg : 'Reg -> Doc) (sym : Sym<'Reg>) : Doc =
          match sym with
          | Reg r -> pReg r
  