The aggregates (columns of x) are grouped by a holding indicator.
Within each holding group, the number of unique groups (output) is set to be equal.
Details
A representative within the holding group is used to calculate output by Ncontributors.
The one with maximal column sum of x is chosen as the representative.
Normally this will be an aggregate representing the holding group total.
When holdingInd is NULL (default), the function is equivalent to Ncontributors.
