anchor.diff:  
   module Orelang =
     type Expr = Nil
  +            | Boolean of bool
               | Number of decimal
               | Symbol of string
               | Function of string * Expr list
  +    with
  +      member this.ToSymbol() =
  +        match this with
  +        | Symbol s -> Some s
  +        | _ -> None
   
     type ErrorKind = ParseError of string
   
  
positive.code:     ParseFromString <| System.IO.File.ReadAllText path
  
    type Engine() =
      let env = new System.Collections.Generic.Dictionary<string, Expr>()
  
      member private this.getValue k =
        match env.TryGetValue(k) with
  
negative.code: // Program.fs
  
  open System.IO
  open System.Collections.Generic
  
  type Result<'t, 'e> = OK of 't | Error of 'e
  
  