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

Must:
  'must' xpath=QUOTED_STRING (';'|'{'
    (description += Description |
     errorMessage += ErrorMessage |
     errorAppTag += ErrorAppTag | 
     reference += Reference |
     subelements +=FromExtension)* 
                
  '}')
;

/**
 * see RFC 6.1.3  (rfc6020#section-6.1.3)
 * 
 * A String can be surrounded with single or double quotes and broken over lines using a +
 */
terminal QUOTED_STRING  : 
      (('"' ( '\\' . /* 'b'|'t'|'n'|'f'|'r'|'u'|'"'|"'"|'\\' */ | !('\\'|'"') )* '"') |
      ("'" ( '\\' . /* 'b'|'t'|'n'|'f'|'r'|'u'|'"'|"'"|'\\' */ | !('\\'|"'") )* "'"))
      (' '|'\t'|'\n'|'\r')*
      
      ('+'(' '|'\t'|'\n'|'\r')*
        (('"' ( '\\' . /* 'b'|'t'|'n'|'f'|'r'|'u'|'"'|"'"|'\\' */ | !('\\'|'"') )* '"') |
        ("'" ( '\\' . /* 'b'|'t'|'n'|'f'|'r'|'u'|'"'|"'"|'\\' */ | !('\\'|"'") )* "'"))
        (' '|'\t'|'\n'|'\r')*
        )*
    ;