Question: How to use plots[animate] with RootLocusPlot?

Hello

Since this is my first attempt to use plots[animate], please forgive if my question is silly.  

I want to use plots[animate] with RootLocusPlot to show the effect of variable.   Here is my attempt.

num:=10*(s+alpha);den:=s*(s^2+4*s+8);
plots[animate](RootLocusPlot,[NewSystem(subs(alpha=a,num/den))],a=1..10);

Of course, when issuing the commands, an error comes out:  

Error, (in plots/animate) symbolic value(s) in model: a
 

• Although I could define sys using NewSystem with a symbolic value, alpha, I have no idea how to assign a value for alpha.  subs(alpha=2,..) does not seem to work.
• If I remove the square brackets in the second argument of plots[animate], a different msg comes out.  Why do I need (or not) them? 

Many thanks.