matlab在运行中显示警告：在未来版本中被删除，要创建首先要建立符号变量，怎么更改程序

警告是这样的：
警告: Support of strings that are not valid variable names or define a number will be removed in a future
release. To create symbolic expressions, first create symbolic variables and then use operations on them.

In sym>convertExpression (line 1536)
In sym>convertChar (line 1441)
In sym>tomupad (line 1198)
In sym (line 177)
In work4_1_2jia3jia4 (line 4)

然后程序是这样的：
sym t;

f1=exp(-t)sym('heaviside(t)');
F1=fourier(f1);
f2=exp(-2t)sym('heaviside(t)');
F2=fourier(f2);
F3=F1F2;
f=ifourier(F3,t);

subplot(3,1,1)
ezplot(t,f1)
grid on
title('f1')
subplot(3,1,2)
ezplot(t,f2)
grid on;
title('f2')
subplot(3,1,3)
ezplot(t,f)
grid on
title('f')

之前搜过有类似问题，说是版本问题可能不承认这种用法，跟着改但还是没解决问题( •̥́ ˍ •̀ू )有没有人能告诉我咋改呀