<> rule0 data
<> rule1 data-1
<> rule2
<> rule3 data-10
<> rule4 data-100
?(?-) (Undefine an empty rule)
>< rule2
?(?-) (Undefine the last rule)
>< rule4
?(?-) (Undefine a non-existant rule)
>< rule5
a sample program