diff --git a/cii/CMakeLists.txt b/cii/CMakeLists.txt
index 391f0eb5..1f34c229 100644
--- a/cii/CMakeLists.txt
+++ b/cii/CMakeLists.txt
@@ -12,6 +12,7 @@ set(SOURCE_FILES
     src/hash.c
     src/list.c
     src/mem.c
+    src/set.c
     src/table.c
     )
 set(HEADER_FILES
@@ -21,6 +22,7 @@ set(HEADER_FILES
     include/cii/hash.h
     include/cii/list.h
     include/cii/mem.h
+    include/cii/set.h
     include/cii/table.h
     )
 
diff --git a/cii/include/cii/set.h b/cii/include/cii/set.h
new file mode 100644
index 00000000..e768dd1c
--- /dev/null
+++ b/cii/include/cii/set.h
@@ -0,0 +1,143 @@
+// Copyright (c) 2023 Xu Shaohua <shaohua@biofan.org>. All rights reserved.
+// Use of this source is governed by GNU General Public License
+// that can be found in the LICENSE file.
+
+#ifndef CII_SET_H_
+#define CII_SET_H_
+
+#include <stddef.h>
+#include <stdbool.h>
+
+typedef struct set_s set_t;
+
+typedef int (*cmp_func)(const void* x, const void* y);
+typedef size_t (*hash_func)(const void* x);
+
+/**
+ * Allocates, initializes and returns a new set.
+ *
+ * If |cmp| is the null pointer, the members are assumed to be atoms; two members
+ * x and y are assumed identical if x == y.
+ * If |hash| is the null pointer, a suitable hash function is provided for atoms.
+ *
+ * May raise kMemFailed.
+ *
+ * @param size_hint is an estimate of the number of members the set is expected
+ * to contain, accurate values of hint may improve performance.
+ * @param cmp used to compare two members
+ * @param hash to map members onto unsigned integer.
+ * @return
+ */
+extern set_t* set_new(size_t size_hint, cmp_func cmp, hash_func hash);
+
+/**
+ * Deallocates set and assigns it the null pointer.
+ *
+ * The members are not deallocated.
+ *
+ * @param set
+ */
+extern void set_free(set_t** set);
+
+/**
+ * Returns number of items in set.
+ *
+ * @param set
+ * @return
+ */
+extern size_t set_length(set_t* set);
+
+/**
+ * Check whether |member| is contained in |set|.
+ *
+ * @param set
+ * @param member
+ * @return
+ */
+extern bool set_member(set_t* set, const void* member);
+
+/**
+ * Adds a |member| into |set|, unless it is already there.
+ *
+ * May raise kMemFailed.
+ *
+ * @param set
+ * @param member
+ */
+extern void set_put(set_t* set, const void* member);
+
+/**
+ * If |set| contains |member|, remove member from |set| and returns member.
+ * Else returns null pointer.
+ *
+ * @param set
+ * @param member
+ * @return
+ */
+extern void* set_remove(set_t* set, const void* member);
+
+/**
+ * Calls |apply| for each member of set.
+ *
+ * @param set
+ * @param apply
+ * @param user_data
+ */
+extern void set_map(set_t* set,
+                    void apply(const void* member, void* user_data),
+                    void* user_data);
+
+/**
+ * Returns a pointer to an N+1 element array that holds N elements of set in an
+ * arbitrary order.
+ *
+ * Clients must arrange to deallocate the return array.
+ *
+ * May raise kMemFailed.
+ *
+ * @param set
+ * @param end is assigned to N+1 element of the array, which is often the null pointer.
+ * @return
+ */
+extern void** set_to_array(set_t* set, void* end);
+
+/**
+ * Returns union of two sets. s + t.
+ *
+ * Raise runtime error if both |s| and |t| are null pointers.
+ *
+ * Note that both |s| and |t| share the same compare function and hash function.
+ *
+ * May raise kMemFailed.
+ *
+ * @param s
+ * @param t
+ * @return
+ */
+extern set_t* set_union(set_t* s, set_t* t);
+
+/**
+ * Return intersection of two sets. s * t.
+ * @param s
+ * @param t
+ * @return
+ */
+extern set_t* set_intersection(set_t* s, set_t* t);
+
+/**
+ * Returns s - t.
+ * @param s
+ * @param t
+ * @return
+ */
+extern set_t* set_minus(set_t* s, set_t* t);
+
+/**
+ * Returns s / t.
+ * @param s
+ * @param t
+ * @return
+ */
+extern set_t* set_diff(set_t* s, set_t* t);
+
+#endif //CII_SET_H_
diff --git a/cii/src/set.c b/cii/src/set.c
new file mode 100644
index 00000000..2752657a
--- /dev/null
+++ b/cii/src/set.c
@@ -0,0 +1,5 @@
+// Copyright (c) 2023 Xu Shaohua <shaohua@biofan.org>. All rights reserved.
+// Use of this source is governed by GNU General Public License
+// that can be found in the LICENSE file.
+
+#include "cii/set.h"
\ No newline at end of file