def relationships
      r = Relationships.new
      @worksheets.each do |sheet|
        r << Relationship.new(sheet, WORKSHEET_R, WORKSHEET_PN % (r.size+1))
      end
      pivot_tables.each_with_index do |pivot_table, index|
        r << Relationship.new(pivot_table.cache_definition, PIVOT_TABLE_CACHE_DEFINITION_R, PIVOT_TABLE_CACHE_DEFINITION_PN % (index+1))
      end
      r << Relationship.new(self, STYLES_R,  STYLES_PN)
      if use_shared_strings
          r << Relationship.new(self, SHARED_STRINGS_R, SHARED_STRINGS_PN)
      end
      r
    end
The workbook relationships. This is managed automatically by the workbook
@return [Relationships]