anchor.diff:  
   // Print all data points
   for record in doc.Array do
  -  if record.Value <> null then
  -    printfn "%d: %f" (int record.Date) (float record.Value)
  -
  -(**
  -When printing the data points, some of them might be missing. Previously, this was handled
  -using the `option` type, but in this case, the type is `string` and F# strings can have 
  -`null` as a value, so the provider just returns a `string` which may be `null`. We can easily
  -test that and print only available data.
  +  record.Value.Number |> Option.iter (fun value ->
  +    printfn "%d: %f" record.Date value)
  +
  +(**
  +When printing the data points, some of the values might be missing (in the input, the value
  +is `null` instead of a valid number). This is another example of a heterogeenous type - 
  +the type is either `Number` or some other type (representing `null` value). This means
  +that `record.Value` has a `Number` property (when the value is a number) and we can use
  +it to print the result only when the data point is available.
   
   ## Parsing Twitter stream
   
  
positive.code: 
  #r "../bin/FSharp.Data.dll"
  open FSharp.Data
  
  (**
  ### Inferring type from sample
  
  
negative.code:  * [F# Data: JSON Parser and Reader](JsonValue.html) - provides more information about 
     working with JSON values dynamically.
  
  *)