0. #lang racket
1. (require syntax/parse)
2.  
3. (syntax-parse #'(#:a #:b #:c)
4.   [((~or (~once (~and a #:a))
5.          (~once (~and b #:b))
6.          (~once (~and #:c
7.                       (~post (~fail #:when (and (attribute a)
8.                                                 (attribute b)
9.                                                 (attribute c)))))))
10.     ...)
11.    #t])

=>

eval:2:0: attribute: not bound as a pattern variable

at: a

in: (attribute a)