[BUG]多元多项式的乘法
>> :mode polyn
Switch into polynomial mode.
>> symbol(x,y)
out> x is a variable for polynomial.
out> y is a variable for polynomial.
------------------------
>> (x+y)(x-y)
in> (x+y)*(x-y)
out> +1x^2-1xy+1xy-1y^2
------------------------
>> (x+y)(x-y)+0
in> (x+y)*(x-y)+0
out> +1x^2+0-1y^2+0
------------------------
>> (x^2+xy+y^2)(x-y)
in> (x^2+xy+y^2)*(x-y)
out> +1x^3-1x^2y+1x^2y+0x^y^-1y^3
------------------------
>> (x^2+xy+y^2)(x-y)+0
in> (x^2+xy+y^2)*(x-y)+0
out> +1x^3+0-1y^3
------------------------
>>