anchor.diff:      let runTerm (ctx: Z3.Context) term =
           let solver = ctx.MkSimpleSolver()
           solver.Assert [| term |]
  -        solver.Check [||]
  +        solver.Check [||]
positive.code:         function
          | Expr.Bool b -> boolToZ3 reals toStr ctx b :> Z3.Expr
          | Expr.Int a -> arithToZ3 reals toStr ctx a :> Z3.Expr
  
  /// <summary>
  ///     Z3 invocation.
  
negative.code:             | ASub xs -> ctx.MkSub (xs |> Seq.map az |> Seq.toArray)
              | AMul xs -> ctx.MkMul (xs |> Seq.map az |> Seq.toArray)
              | ADiv (x, y) -> ctx.MkDiv (az x, az y)
              // TODO(CaptainHayashi): refuse AMod when reals is true.
              | AMod (x, y) -> ctx.MkMod (az x :?> Z3.IntExpr, az y :?> Z3.IntExpr) :> Z3.ArithExpr
          az
  
      /// Converts a Starling Boolean expression to a Z3 ArithExpr.
  