diff --git a/mux21.txt b/mux21.txt
new file mode 100644
index 0000000..3dd7a10
--- /dev/null
+++ b/mux21.txt
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+DEF MUX21
+    PORT IN I0
+    PORT IN I1
+    PORT IN S
+    PORT OUT Z
+
+    NET A
+    NET B
+    NET C
+
+    INST NAND1 NAND S  S  A
+    INST NAND2 NAND S  I1 B
+    INST NAND3 NAND I0 A  C
+    INST NAND4 NAND B  C  Z
+ENDDEF