Description of sort_op

SORT path_expr 
   ( REVERSE 
     | ( [ REMOVE NULLS ] ORDER BY ( path_expr [ ASC | DESC ] )
             [ , ( path_expr [ ASC | DESC ] ) ]...
     | [ ASC | DESC ] [ UNIQUE ] [ REMOVE NULLS ] 
       )
    )  
  [ {  IGNORE | ERROR | NULL } ON MISSING ]
  [ {  IGNORE | ERROR | NULL } ON MISMATCH ]
  [ {  IGNORE | ERROR } ON EMPTY ]
  [ {  IGNORE | ERROR } ON ERROR ]