def p_expression_binop(t):
    """expression : expression PLUS expression
                  | expression MINUS expression
                  | expression TIMES expression
                  | expression DIVIDE expression
    """
    if t[2] == '+':
        t[0] = t[1] + t[3]
    elif t[2] == '-':
        t[0] = t[1] - t[3]
    elif t[2] == '*':
        t[0] = t[1] * t[3]
    elif t[2] == '/':
        t[0] = t[1] / t[3]
