docs: private int[] countItemSupport(final Relation<BitVector> relation, final int dim) {
      final int[] counts = new int[dim];
      FiniteProgress prog = LOG.isVerbose() ? new FiniteProgress("Finding frequent 1-items", relation.size(), LOG) : null;
      for(DBIDIter iditer = relation.iterDBIDs(); iditer.valid(); iditer.advance()) {
        SparseFeatureVector<?> bv = relation.get(iditer);
        // TODO: only count those which satisfy minlength?
        for(int it = bv.iter(); bv.iterValid(it); it = bv.iterAdvance(it)) {
          counts[bv.iterDim(it)]++;
        }
        LOG.incrementProcessed(prog);
      }
      LOG.ensureCompleted(prog);
      return counts;
    }