anchor.diff:  
     /// Represents a mutable variable, called a stream variable, that generates a
     /// stream of values as a side-effect.
  +#if DOC
  +  ///
  +  /// The difference between a stream variable and a stream source is that a
  +  /// stream variable cannot be closed and always has a value.  Stream variables
  +  /// are one way to represent state, or the model, manipulated by a program
  +  /// using streams.
  +  ///
  +  /// Note that there are no special hidden mechanisms involved in the
  +  /// implementation of stream variables.  You can easily implement similar
  +  /// imperative mechanisms outside of the stream library.
  +#endif
     type Var<'x>
   
     /// Operations on stream variables.
  
positive.code: 
    /// Represents an imperative source of a stream of values called a stream
    /// source.
    type Src<'x>
  
    /// Operations on stream sources.
  
negative.code:   /// streams.  For example,
    ///
    ///> let fibs: Stream<BigInteger> =
    ///>   let rec lp f0 f1 = cons f0 << delay <| fun () -> lp f1 (f0 + f1)
    ///>   lp 0I 1I
    ///
    /// is the stream of all fibonacci numbers.
  