EXPR = TERM | EXPR '+' TERM | EXPR '-' TERM;
TERM = FACTOR | TERM '*' FACTOR | TERM '/' FACTOR;
FACTOR = NUMBER | '(' EXPR ')';
NUMBER = DIGIT | NUMBER DIGIT;
DIGIT = '0' | '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' | '9';