arith
extern Cell *arith(Node **, int);
r1 = arith(op1, r1, expres(pri[op1-OR], 0));
| expr ADD expr { $$ = arith(ADD, $1, $3); }
| expr SUBT expr { $$ = arith(SUBT, $1, $3); }
| expr MULT expr { $$ = arith(MULT, $1, $3); }
| expr DIV expr { $$ = arith(DIV, $1, $3); }
| expr REM expr { $$ = arith(REM, $1, $3); }