直接在matlab命令窗口敲入以下命令，然后按回车执行，显示下面的，是怎么回事？
ans =

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??? Undefined function or variable 'c'.




var y,c,k,h,a,b;
varexo e,u;
parameters beta,alpha,delta,theta,psi,rho,tau;
alpha=0.36;
rho=0.95;
tau=0.025;
beta=0.99;
delta=0.025;
psi=0;
theta=2.95;
phi=0.1;
model;
c*theta*h^(1+psi)=(1-alpha)*y;
k=beta*(((exp(b)*c)/(exp(b(+1))*c(+1)))*(exp(b(+1))*alpha*y(+1)+(1-delta)*k));
y=exp(a)*(k(-1)^alpha)*(h^(1-alpha));
k=exp(b)*(y-c)+(1-delta)*k(-1);
a=rho*a(-1)+tha*b(-1)+e;
b=tha*a(-1)+rho*b(-1)+u;
end;
intval;
y=1.08068253095672;
c=0.80359242014163;
h=0.29175631001732;
k=11.08360443260358;
a=0;
b=0;
e=0;
u=0;
end;
shocks;
var e; stderr 0.009;
var u; stderr 0.009;
var e, u=phi*0.009*0.009;
end;
stoch_simul(periods=2000, drop=200);