anchor.diff:  
   module Arachne.Core
   
  +open System.Globalization
   open System.Text
   open FParsec
   
  
positive.code:             match tryParse mapping s with
              | Choice1Of2 x -> x
              | Choice2Of2 e -> failwith e
  
  (* Grammar *)
  
  
negative.code: 
   and Format<'a> =
      'a -> StringBuilder -> StringBuilder
  
  (* Mapping *)
  
  