gen_arth
struct arth *gen_arth(int, struct arth *, struct arth *);
| arth '+' arth { $$ = gen_arth(BPF_ADD, $1, $3); }
| arth '-' arth { $$ = gen_arth(BPF_SUB, $1, $3); }
| arth '*' arth { $$ = gen_arth(BPF_MUL, $1, $3); }
| arth '/' arth { $$ = gen_arth(BPF_DIV, $1, $3); }
| arth '&' arth { $$ = gen_arth(BPF_AND, $1, $3); }
| arth '|' arth { $$ = gen_arth(BPF_OR, $1, $3); }
| arth LSH arth { $$ = gen_arth(BPF_LSH, $1, $3); }
| arth RSH arth { $$ = gen_arth(BPF_RSH, $1, $3); }