docs: private int dimensionality(Relation<O> rel) {
      // Explicit:
      if(idim >= 0) {
        return idim;
      }
      // Cast to vector field relation.
      @SuppressWarnings("unchecked")
      final Relation<NumberVector> frel = (Relation<NumberVector>) rel;
      int dim = RelationUtil.dimensionality(frel);
      if(dim < 1) {
        throw new AbortException("When using KDEOS with non-vectorspace data, the intrinsic dimensionality parameter must be set!");
      }
      return dim;
    }