Simulate regARIMA with AR, SMA and Betas ...Problem with presamples!!

Hello,

I have a model, built with regARIMA, with the following:

Intercept =0; AR(1); AR(2); AR(3); SMA(12); Beta1; Beta2; Beta3

I need to simulate 10.000 paths. When I run:

YYY = simulate(EstMdl,100,'NumPaths',10000,'E0',e0,'U0',u0,'X',XF1,'Y0',Y1);

The problem is that I cannot pass 'Y0' since "simulate" regARIMA accept only 'E0' and 'U0'. My question is... is it correct? How does it calculate the value of AR(1); AR(2); AR(3) whithout having past values of Y ??