[racket] Please help with syntax

 I need to define syntax set!-values*, that will ignore _ in value place.

Such as
(define a #f)
(set!-values* (_ _ a) (values 1 2 3)) ; should set a to 3.

But I have two problems:
1. _ don't want to pattern match in syntax case. If I make it literal, I can't match "anything else".
2. How to omit extra values. It seems that in any case I have to assign them to something...

Can you help me? I hope, that my problem in fact not hard, but I have not enough experience to solve it.

-- 
Roman Klochkov
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