class RubyXL::OOXMLRelationshipsFile

def add_relationship(obj)
  return if obj.nil? || !defined?(obj.class::REL_TYPE) || (obj.respond_to?(:empty?) && obj.empty?)
  relationships << RubyXL::Relationship.new(:id     => "rId#{relationships.size + 1}",
                                            :type   => obj.class::REL_TYPE,
                                            :target => obj.xlsx_path.relative_path_from(owner.xlsx_path.dirname))
end