in src/main/scala/InvoiceHelper.scala [15:18]
def identifyUnpaidInvoices(invoices: List[Invoice]): String \/ List[Invoice] = {
val unpaidInvoices = invoices.filter(invoice => invoice.balance > 0)
if (unpaidInvoices.isEmpty) "Failed to find an unpaid invoice on account".left else unpaidInvoices.right
}