PROGRAM
   BLOCK
      begin   
      LIST
         STATEMENT
            ASSIGNMENT
               identifier   x
               :=   
               EXPRESSION
                  number   12
         TAIL
            ;   
            LIST
               STATEMENT
                  ASSIGNMENT
                     identifier   y
                     :=   
                     EXPRESSION
                        number   3
               TAIL
                  ;   
                  LIST
                     STATEMENT
                        BLOCK
                           begin   
                           LIST
                              STATEMENT
                                 ASSIGNMENT
                                    identifier   number
                                    :=   
                                    EXPRESSION
                                       identifier   x
                              TAIL
                                 ;   
                                 LIST
                                    STATEMENT
                                       ASSIGNMENT
                                          identifier   y
                                          :=   
                                          EXPRESSION
                                             number   44
                                    TAIL
                           end   
                     TAIL
                        ;   
                        LIST
                           STATEMENT
                              ASSIGNMENT
                                 identifier   number
                                 :=   
                                 EXPRESSION
                                    identifier   y
                           TAIL
      end   

Parsing has succeeded