'expression : expression PLUS expression
| expression MINUS expression
| expression TIMES expression
| expression DIVIDE expression
| expression EXP expression'

def p_expression_binop(self, p):

    if (p[2] == '+'):
        p[0] = (p[1] + p[3])
    elif (p[2] == '-'):
        p[0] = (p[1] - p[3])
    elif (p[2] == '*'):
        p[0] = (p[1] * p[3])
    elif (p[2] == '/'):
        p[0] = (p[1] / p[3])
    elif (p[2] == '**'):
        p[0] = (p[1] ** p[3])
