code: protected static double[][] means(List<? extends DBIDs> clusters, double[][] means, Relation<? extends NumberVector> relation) {
      if(TypeUtil.SPARSE_VECTOR_FIELD.isAssignableFromType(relation.getDataTypeInformation())) {
        @SuppressWarnings("unchecked")
        Relation<? extends SparseNumberVector> sparse = (Relation<? extends SparseNumberVector>) relation;
        return sparseMeans(clusters, means, sparse);
      }
      return denseMeans(clusters, means, relation);
    }
docs: Returns the mean vectors of the given clusters in the given database.
  
  @param clusters the clusters to compute the means
  @param means the recent means
  @param relation the database containing the vectors
  @return the mean vectors of the given clusters in the given database