Add comments.

mux2.sv

@@ -1,6 +1,8 @@
+/* A two-input multiplexer.
+ */
 module mux2 #(width=32)
 			 (input logic [width-1:0] left, right,
 			  input logic select,
 			  output logic [width-1:0] out);
 	assign out = select ? right : left;
-endmodule
+endmodule