Message

Mandate reference MUST be provided for direct debit.

Context
cac:PaymentMeans[some $code in tokenize('49 59', '\s') satisfies normalize-space(cbc:PaymentMeansCode) = $code]
Test
cac:PaymentMandate/cbc:ID
Usage
/ ubl:Invoice / cac:PaymentMeans / cac:PaymentMandate / cbc:ID