Question: No polynom in x??


I have another problem and maybe someone can help me.
When I enter the code: 

with  alias(tau=RootOf(T^2+2T+2)) ;
then Maple gives me the output false???
Why is it false??
Can someone help me?
How do I tell Maple that for example f:= x^2+tau*x is a Polynom with x=variable, tau= RootOf?
Thank you!!

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