Add "Custom function" to group header cells (with Sum, Avg, Range, Max...)

The group header cells, or “Summary cells” have

• Sum
• Average
• Median
• Min
• Max
  etc etc

What about adding a “Custom function” option that would allow to enter a function such as the ones you can find in C# Linq? (see for instance Enumerable.Aggregate or Enumerable.ForEach)

Admittedly this is a huge amount of work. But I thought I would suggest it, see who might need that too.