Question: converting tan(z) to sum form with assumption


I am trying to convert tan(z) to sum form using the convert(...,Sum) command and using some information from FunctionAdvisor.
The second operand of FunctionAdvisor( sum_form, tan) gives me the domain of validity.
I would like to use this info during the conversion as assumption, but it doesn't work for me.
(Of course, the first operand is the required sum form, I am just playing with maple).
Here is my problem:

a:=FunctionAdvisor( sum_form, tan);
 convert(tan(z),Sum) assuming op(2,a); #it doesn't work for me
 convert(tan(z),Sum) assuming And(abs(z) < 1/2*Pi); # it works 

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