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diff --git a/books/workshops/1999/mu-calculus/book/sets.lisp b/books/workshops/1999/mu-calculus/book/sets.lisp
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+;; 2014-01-02: Changed from SETS by Jared to avoid conflict with std/sets
+;; 2014-02-15: Changed back since std/sets got renamed to set
+(in-package "SETS")
+
+; Checks for set containment, i.e., is a in X?
+(defun in (a X)
+  (cond ((endp X) nil)
+        ((equal a (car X)) t)
+        (t (in a (cdr X)))))
+
+; Subset, i.e., X =< Y
+(defun =< (X Y)
+  (cond ((endp X) t)
+        (t (and (in (car X) Y)
+                (=< (cdr X) Y)))))
+
+; Set equality, i.e., X == Y
+(defun == (X Y)
+  (and (=< X Y)
+       (=< Y X)))
+
+; set union, i.e., X U Y
+(defun set-union (X Y)
+  (if (endp X)
+      Y
+    (cons (car X) (set-union (cdr X) Y))))
+
+; Set intersection, i.e., X & Y
+(defun intersect (X Y)
+  (cond ((endp X) nil)
+        ((in (car X) Y)
+         (cons (car X) (intersect (cdr X) Y)))
+        (t (intersect (cdr X) Y))))
+
+; Set minus, i.e.,  X - Y
+(defun minus (X Y)
+  (cond ((endp X) nil)
+        ((in (car X) Y)
+         (minus (cdr X) Y))
+        (t (cons (car X) (minus (cdr X) Y)))))
+
+; complement set X, i.e., U - X
+(defun set-complement (X U)
+  (minus U X))
+
+; Remove duplicates in X
+(defun remove-dups (X)
+  (cond ((endp X) nil)
+        ((in (car X) (cdr X))
+         (remove-dups (cdr X)))
+        (t (cons (car X)
+                 (remove-dups (cdr X))))))
+
+; Cardinality of X, i.e., |X|
+(defun cardinality (X)
+  (len (remove-dups X)))
+
+(defun s< (X Y)
+  (and (=< X Y) (not (=< Y X))))
+