syntax/parse equality constraint

#lang racket

(require (for-syntax syntax/parse))

(define-syntax (trivial-example stx)
  (syntax-parse stx
    [(_ (a . b))
     #''(b . a)]))

(trivial-example (c . d))

#;(define-syntax (equality-constraint-fail stx)
  (syntax-parse stx
    [(_ (a . a))
     #''(a . a)]))
#;(equality-constraint-fail (e . f))