def relationships
      rels = Axlsx::Relationships.new
      rels << Relationship.new(self, WORKBOOK_R, WORKBOOK_PN)
      rels << Relationship.new(self, CORE_R, CORE_PN)
      rels << Relationship.new(self, APP_R, APP_PN)
      rels.lock
      rels
    end
def relationships
      rels = Axlsx::Relationships.new
      rels << Relationship.new(self, WORKBOOK_R, WORKBOOK_PN)
      rels << Relationship.new(self, CORE_R, CORE_PN)
      rels << Relationship.new(self, APP_R, APP_PN)
      rels.lock
      rels
    end
Creates the relationships required for a valid xlsx document
 @return [Relationships]
 @private