hspice .vec with .alter

[zhouk]

I am using .vec to include vector stimuli in my hspice simulation. For example,

.vec "stimuli.hvec"

I need to test multiple stimuli in my hspice simulation. So I tried:

.alter
.vec "stimuli1.hvec"

.alter
.vec "stimuli2.hvec"

........

But the program comes back with errors like:

multiple drivers for node xxxx given in VCD/VEC stimulus

Any solutions?