diff --git a/words/main.rkt b/words/main.rkt
index b429852..6fe3aec 100644
--- a/words/main.rkt
+++ b/words/main.rkt
@@ -1,8 +1,13 @@
 #lang debug racket/base
 (require racket/list
+         racket/set
          "index.rkt")
 (provide make-words)
 
+(define (string->set str)
+  (for/seteqv ([c (in-string (or str ""))])
+              (char-downcase c)))
+
 (define (make-words #:letters [letters-arg #f]
                     #:mandatory [mandatory #f]
                     #:omit [omit #f]
@@ -13,33 +18,26 @@
                     #:proper-names [proper-names? #f]
                     #:count [count 10]
                     #:case [casing #f])
-  (define letters (or letters-arg "abcdefghijklmnopqrstuvwxyz"))
-  (define mandatory-cs
-    (if (or mandatory combo)
-        (remove-duplicates
-         (for/list ([c (in-string (string-append (or mandatory "") (or combo "")))])
-                   (char-downcase c)) char=?) null))
-  (define forbidden-cs
-    (remove-duplicates
-     (for/list ([c (in-string (or omit ""))])
-      (char-downcase c)) char=?))
+    
+  (define letter-set (string->set (or letters-arg "abcdefghijklmnopqrstuvwxyz")))
+  (define mandatory-set (set-union (string->set mandatory) (string->set combo)))
+  (define omitted-set (string->set omit))
+  
   (define letter-cs-charidx
     (word->charidx
      (list->string
-      (remove-duplicates
-       (append (if letters
-                   (for/list ([c (in-string letters)])
-                             (char-downcase c))
-                   null)
-               mandatory-cs)
-       char=?))))
+      (set->list
+       (set-subtract (set-union letter-set mandatory-set) omitted-set)))))
+  
   (define caser (case casing
                   [(up upcase upper uppercase) string-upcase]
                   [(title titlecase) string-titlecase]
                   [(down downcase lower lowercase) string-downcase]
                   [else values]))
+  
   (define min-length (or min-length-arg 0))
   (define max-length (or max-length-arg +inf.0))
+  
   (for*/fold ([word-acc null]
               [count-acc 0]
               #:result word-acc)
@@ -52,27 +50,22 @@
                       ;; between min and max length
                       ((if (<= min-length max-length) <= >=) min-length (word-rec-length rec) max-length)
                       ;; word contains each mandatory char, case-insensitive
-                      (for/and ([mc (in-list mandatory-cs)])
+                      (for/and ([mc (in-set mandatory-set)])
                                (word-charidx . contains-char? . mc))
-                      ;; word contains only letters + mandatory, case-insensitive
+                      ;; word contains only (letters + mandatory) - forbidden,
+                      ;; case-insensitive
                       (for/and ([wc (in-list (map char-downcase (charidx->chars word-charidx)))])
                                (letter-cs-charidx . contains-char? . wc))
                       (or (not combo)
                           (regexp-match (string-downcase combo) word))
-                      ;; word does not contain forbidden characters
-                      (for/and ([fc (in-list forbidden-cs)])
-                        (not (word-charidx . contains-char? . fc)))
                       ;; maybe only proper names
-                      (if proper-names?
-                          (capitalized? word-charidx)
-                          (not (capitalized? word-charidx)))
+                      ((if proper-names? values not) (capitalized? word-charidx))
                       ;; maybe hide plurals
-                      (or (not hide-plurals?)
-                          (not (word-rec-plural? rec)))))
+                      (or (not hide-plurals?) (not (word-rec-plural? rec)))))
     (values (cons (caser word) word-acc) (add1 count-acc))))
 
 (module+ test
   (require rackunit)
   (time (make-words))
   (check-equal? (sort (make-words #:mandatory "xyz" #:combo #false #:letters "etaoinshrdluw") string<?)
-                  '("azoxy" "dysoxidize" "isazoxy" "oxytonize" "rhizotaxy" "zootaxy")))
\ No newline at end of file
+                '("azoxy" "dysoxidize" "isazoxy" "oxytonize" "rhizotaxy" "zootaxy")))
\ No newline at end of file