def each_matching_line(marc_record)
      marc_record.fields(interesting_tags).each do |field|

        specs_covering_field(field).each do |spec|
            yield(field, spec, self)
        end

      end
    end
def each_matching_line(marc_record)
      marc_record.fields(interesting_tags).each do |field|

        specs_covering_field(field).each do |spec|
            yield(field, spec, self)
        end

      end
    end
Yields a block for every line in source record that matches
 spec. First arg to block is MARC::DataField or ControlField, second
 is the MarcExtractor::Spec that it matched on. May take account
 of options such as :alternate_script

 Third (optional) arg to block is self, the MarcExtractor object, useful for custom
 implementations.