anchor.diff:          | Sigmoid    -> sigmoid x
           | SoftMax    -> softmax x
           | LogSoftmax -> logSoftmax x
  +        | Relu       -> relu x
           | Identity   -> id x
   
   /// Regularization expressions.
  
positive.code:         | SoftMax
          /// logarithm of soft-max transfer function
          | LogSoftmax
          /// no transfer function
          | Identity
  
  
negative.code:         let c = x |> Expr.maxKeepingAxis 1
          x - c - log (Expr.sumKeepingAxis 1 (exp (x - c))) 
  
      /// Rectifier Unit function: max(x, 0)
      let relu (x: ExprT) =
          x |> Expr.zerosLike |> Expr.maxElemwise x
  
      /// applies the specified activation function
      let apply af x =
          match af with
  