Constructs the aggregation matrix Au for the sub-hierarchy composed only of the upper variables. This matrix relates the "upper uppers" to the "lowest uppers" (the most disaggregated level of the upper hierarchy).
.get_Au(A, lowest_rows = NULL)Aggregation matrix for the upper sub-hierarchy, or NULL if all uppers are at the lowest level.
Aggregation matrix (must be hierarchical)
Optional. Integer vector of row indices corresponding to
the lowest upper level. If NULL, computed using .lowest_lev(A).