anchor.diff:      [ "key", GroupAggregation.GroupKey; "count-all", GroupAggregation.CountAll ]
   
     let winUnaryOps = 
  -    [ "mean", WindowAggregation.Mean; 
  +    [ "mean", WindowAggregation.Mean; "sum", WindowAggregation.Sum
         "min", WindowAggregation.Min; "max", WindowAggregation.Max ]
     let winNullaryOps = 
       [ "first-key", WindowAggregation.FirstKey; "last-key", WindowAggregation.LastKey
  
positive.code:   | SortBy of (string * SortDirection) list
    | GroupBy of string list * GroupAggregation list
    | WindowBy of string * int * WindowAggregation list
    | FilterBy of FilterOperator * FilterCondition list
    | Paging of Paging list
    | Empty
  
negative.code:   | Min of string
    | Max of string
    | Mean of string
    | Sum of string
    | FirstKey
    | LastKey
    | MiddleKey
  