A data frame with 100 observations, 5 variables,
  and a special "cells" attribute:
  - A
 Factor with levels 1, 2, and 3
 
  - B
 Factor with levels 1, 2, and 3
 
  - C
 Factor with levels 1, 2, and 3
 
  - x
 A numeric variable
 
  - y
 A numeric variable
 
In addition, attr(ubds, "cells") consists of a named list of length 27 with the row numbers for
each combination of A, B, C. For example, 
attr(ubds, "cells")[["213"]] has the row numbers corresponding
to levels A == 2, B == 1, C == 3. The entries are ordered by
length, so the first entry is the cell with the lowest frequency.