Function definitions are not permitted at the prompt or in s

Function definitions are not permitted at the prompt or in scripts.请高手帮我看看这段里面的错误
syms G H x0 u0 N T
function [t,xx]=diffstate(G,H,x0,u0,N,T)
xk=x0;u=u0;t=0;
for k=1:N
xk=G*xk+H*u;
x(:k)=x0;
xx=[x0,x];
t=[t,k*T];
end