RFC 7950, "The YANG 1.1 Data Modeling Language", August 2016

Errata ID: 5698
Status: Verified
Type: Editorial
Publication Format(s) : TEXT
Reported By: Andy Bierman
Date Reported: 2019-04-18
Verifier Name: Ignas Bagdonas
Date Verified: 2019-04-24

Section 7.5.4.3 says:

     container interface {
       leaf ifType {
         type enumeration {
           enum ethernet;
           enum atm;
         }
       }
       leaf ifMTU {
         type uint32;
       }
       must 'ifType != "ethernet" or ifMTU = 1500' {
         error-message "An Ethernet MTU must be 1500";
       }
       must 'ifType != "atm" or'
          + ' (ifMTU <= 17966 and ifMTU >= 64)' {
         error-message "An ATM MTU must be 64 .. 17966";
       }
     }

It should say:

     container interface {
       leaf ifType {
         type enumeration {
           enum ethernet;
           enum atm;
         }
       }
       leaf ifMTU {
         type uint32;
       }
       must 'string(ifType) != "ethernet" or ifMTU = 1500' {
         error-message "An Ethernet MTU must be 1500";
       }
       must 'string(ifType) != "atm" or'
          + ' (ifMTU <= 17966 and ifMTU >= 64)' {
         error-message "An ATM MTU must be 64 .. 17966";
       }
     }

Notes:

The intent of the example is for each must-stmt to be false if the ifType leaf does not exist.
However the XPath is incorrect.

From the XPath 1.0 spec, section 3.4, para 5

If one object to be compared is a node-set and the other is a string, then the comparison will be true if and only if there is a node in the node-set such that the result of performing the comparison on the string-value of the node and the other string is true.

The empty node-set is not implicitly converted to an empty string for a = or != comparison.
Instead the string() function must explicitly convert the node-set to a string

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