Trying to create an empty set in Clojure

Question

I have this function called not-elimination which takes an argument and applies the not inference rule which states: (not (not x)) infer x. So for example if my argument is '(not (not a)) then #{a} would be my output. Example 2, Argument: '(not (not (not a))) Output: #{(not a)}

The problem I am running into is the case where my argument is '(not x) which is supposed to return #{} (empty set), but I am getting the error below. Any ideas on what the problem is?

    Execution error (IllegalArgumentException) at microproject2.core/not-elimination (core.clj:7).
Don't know how to create ISeq from: clojure.lang.Symbol

My code:

(ns microproject2.core)

(defn not-elimination [expression]
  (if(not-empty expression)
  (if (= (nth expression 1) "x" )
    (set {})
   (sorted-set-by > (last (last expression))))
  (println "The list is empty")))

Answer 1

Here is an example that works, including some unit tests:

(ns tst.demo.core
  (:use tupelo.core tupelo.test))

(defn not-elimination
  [expr]
  (if (not-empty? expr)
    (if (= (symbol "x") (second expr))
      #{}   ; and empty set, could use `(hash-set)`
      (sorted-set-by > (last (last expr))))
    :the-list-is-empty))

(dotest
  (is= #{} (hash-set)) ; both are equivalent

  (is= :the-list-is-empty (not-elimination '()))
  (is= (hash-set 'x) (not-elimination '(not (not x))))
  (is= #{ '(not x) } (not-elimination '(not (not (not x)))))
  )

It is based on this template project.