anchor.diff:    /// entire window contains fewer than 2 values, the result is missing.
     ///
     /// [category:Expanding windows]
  -  static member expandingStdDev (series:Series<'K, float>) : Series<'K, float> =
  +  static member inline expandingStdDev (series:Series<'K, 'V>) : Series<'K, float> =
       let toStdDev w = 
         if w.nobs < 2.0 then nan
         else w.M2 / (w.nobs - 1.0) |> sqrt
  
positive.code:   /// entire window contains fewer than 3 values, the result is missing.
    ///
    /// [category:Expanding windows]
    static member expandingSkew (series:Series<'K, float>) : Series<'K, float> =
      // population -> sample estimate    
      let toEstSkew w = 
        if w.nobs < 3.0 then nan else
  
negative.code:   /// entire window contains fewer than 4 values, the result is missing.
    ///
    /// [category:Expanding windows]
    static member inline expandingKurt (series:Series<'K, 'V>) : Series<'K, float> =
      // population -> sample estimate
      let toEstKurt w = 
        if w.nobs < 4.0 then nan else
  