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;'<<SPEC'

<!--
Copyright (c) 2012-2024, Chris Pressey, Cat's Eye Technologies.
This file is distributed under a 2-clause BSD license.  See LICENSES/ dir.
SPDX-License-Identifier: LicenseRef-BSD-2-Clause-X-Robin
-->

### `elem?` ###

    -> Tests for functionality "Evaluate Robin Expression (with List)"

`elem?` evaluates its first argument to a value of any type, and its
second argument to obtain a list.  It then evaluates to `#t` if the value
is `equal?` to some element of the list, `#f` otherwise.

    |   (elem? (literal p) (literal (a p e)))
    = #t

    |   (elem? (literal p) (literal (a r k)))
    = #f

    |   (elem? 7 ())
    = #f

    |   (elem? 7 (list 5 (list 6 7) 8))
    = #f

`elem?` can be defined in terms of `find`, in a manner such as:

    (not (empty? (find (fun (x) (equal? x y)) li)))

'<<SPEC'

(define elem? (fun (item li)
  (bind elem?-r (fun (self item li)
    (if (empty? li)
      #f
      (if (equal? item (head li))
        #t
        (self self item (tail li)))))
  (elem?-r elem?-r item li))))