def max
      negative = txns.select { |t| t.amount.negative? }
      negative.empty? ? 0 : negative.max_by(&:id).id
    end
def max
      negative = txns.select { |t| t.amount.negative? }
      negative.empty? ? 0 : negative.max_by(&:id).id
    end
Calculate the maximum transaction ID visible currently in the wallet.
 We go through them all and find the largest number. If there are
 no transactions, zero is returned.