Hi,

I noticed a slight discomfort in using generics while grouping.

GSum<T extends Number & Comparable<T>> could be just GSum<T extends Number>
GAvg<T extends Number & Comparable<T>> could be just GAvg<T extends Number>
GMin<T extends Comparable<T>> should beGMin<T extends Comparable<? super T>>
GMax<T extends Comparable<T>> should be GMax<T extends Comparable<? super T>>

The latter ones would actually be useful for me because I have to dumb down the generics in order to get things working (or typecast the date as java.util.Date)
GroupExpression<java.sql.Date, java.sql.Date> min = GroupBy.min((DatePath) date)