example of equal?/recur that deals with cycles.

equal-with-id.rkt

#lang racket/base

(require data/union-find)

(define (equal?/id x y) 
  (define ufs (make-hasheq))
  (let loop ([x x]
             [y y])
    (cond [(and (identifier? x) (identifier? y))
           (free-identifier=? x y)]
          [else
           (define ux (hash-ref! ufs x (lambda () (uf-new x))))
           (define uy (hash-ref! ufs y (lambda () (uf-new y))))
           (cond
             [(uf-same-set? ux uy)
              #t]
             [else
              (uf-union! ux uy)
              (equal?/recur x y loop)])])))