code: def legacy(wallet, hours: 24)
        raise 'You can\'t add legacy to a non-empty patch' unless @id.nil?
        wallet.txns.each do |txn|
          @txns << txn if txn.amount.negative? && txn.date < Time.now - hours * 60 * 60
        end
      end
docs: Add legacy transactions first, since they are negative and can't
   be deleted ever. This method is called by merge.rb in order to add
   legacy negative transactions to the patch before everything else. They
   are not supposed to be disputed, ever.