Examples ...

begin i:=j; j:=7 end.    /* input */
binop
 79460  OP     :=  link     0  operands 79412
 79412  ID      I  link 79508
 79508  ID      J  link     0
binop
 79652  OP     :=  link     0  operands 79604
 79604  ID      J  link 79700
 79700  NUM     7  link     0
cons
 79652  OP     :=  link     0  operands 79604
     0  NULL
cons
 79460  OP     :=  link 79652  operands 79412
 79652  OP     :=  link     0  operands 79604
makeprogn
 79364  OP  progn  link     0  operands 79460
 79460  OP     :=  link 79652  operands 79412

yyparse result = 0
 79364  OP  progn  link     0  operands 79460

(progn (:= I J)
       (:= J 7))