[MATLAB信号处理] 符号运算求卷积（书中程序敲到电脑里有错误），求大神帮忙

[woshidcp]

题目：符号运算求卷积y(t)=[u(t)-u(t-1)]*[u(t)-u(t-1)]
程序：
syms tao;
t=sym('t','positive');
xt1=sym('Heaviside(t)-Heaviside(t-1)');
xt2=sym('Heaviside(t)-Heaviside(t-1)');
xt_tao=subs(xt1,t,tao)*subs(xt2,t,t-tao);
yt=int(xt_tao,'tao',0,t);
yt=simplify(yt);
ezplot(yt,[0,2])

//运算到yt=int(xt_tao,'tao',0,t);时，提示
Warning: Explicit integral could not be found.
然后无法画图