# File lib/vpim/vcard.rb, line 1394
      def copy(card) # :yields: Field
        card.each do |field|
          fieldname = field.name.upcase
          case
          when [ 'BEGIN', 'END' ].include?(fieldname)
            # Never copy these

          when [ 'VERSION', 'N', 'FN' ].include?(fieldname) && @card.field(fieldname)
            # Copy these only if they don't already exist.

          else
            if block_given?
              field = yield field
            end

            if field
              add_field(field)
            end
          end
        end
      end