diff --git a/src/kyupy/sim.py b/src/kyupy/sim.py
index bdb4d95..3d5de82 100644
--- a/src/kyupy/sim.py
+++ b/src/kyupy/sim.py
@@ -25,14 +25,19 @@ XOR4 = np.uint16(0b0110_1001_1001_0110)
 
 XNOR2, XNOR3, XNOR4 = ~XOR2, ~XOR3, ~XOR4
 
-AO21 = np.uint16(0b1110_1010_1110_1010)
-AO22 = np.uint16(0b1111_1000_1000_1000)
-OA21 = np.uint16(0b1010_1000_1010_1000)
-OA22 = np.uint16(0b1110_1110_1110_0000)
+AO21 = np.uint16(0b1110_1010_1110_1010)  # i0 | (i1 & i2)
+AO22 = np.uint16(0b1111_1000_1000_1000)  # (i0 & i1) | (i2 & i3)
+OA21 = np.uint16(0b1010_1000_1010_1000)  # i0 & (i1 | i2)
+OA22 = np.uint16(0b1110_1110_1110_0000)  # (i0 | i1) & (i2 | i3)
 
 AOI21, AOI22, OAI21, OAI22 = ~AO21, ~AO22, ~OA21, ~OA22
 
-MUX21 = np.uint16(0b1110_0100_1110_0100)
+AO211 = np.uint16(0b1111_1110_1110_1110)  # i0 | i1 | (i2 & i3)
+OA211 = np.uint16(0b1000_1000_1000_0000)  # i0 & i1 & (i2 | i3)
+
+AOI211, OAI211 = ~AO211, ~OA211
+
+MUX21 = np.uint16(0b1100_1010_1100_1010)  # z = i1 if i2 else i0 (i2 is select)
 
 names = dict([(v, k) for k, v in globals().items() if isinstance(v, np.uint16)])