def exists?(details)
      !@wallet.txns.find { |t| t.details.start_with?("#{PREFIX} ") && t.details == details }.nil?
    end
def exists?(details)
      !@wallet.txns.find { |t| t.details.start_with?("#{PREFIX} ") && t.details == details }.nil?
    end
Check whether this tax payment already exists in the wallet.