Selections, continued
Selections usually produce subsets of the cube which have no product structure unless the predicate has certain properties which imply that the selection would necessarily produce a subgrid. Particular distribution of data values can also produce subgrid effects.
If the result R of a selection is not a subgrid, we deflate it to the maximal possible number of dimensions. The number of the dimensions of the deflation is bounded from above by hdim (R) + 1. Certain requirements on the predicates can ensure a bound from below regardless of data. In the worst case we have to deflate R to a one-dimensional object.