Add comments.

mux4.sv

@@ -1,10 +1,13 @@
+/**
+ * A four-input multiplexer.
+ */
 module mux4 #(width=32)
 			 (input logic [width-1:0] first, second, third, fourth,
 			  input logic [1:0] select,
 			  output logic [width-1:0] out);
-			  
+
 	logic [width-1:0] lower, upper;
-	
+
 	mux2 lower_mux(
 		.left(first),
 		.right(second),
@@ -15,10 +18,10 @@ module mux4 #(width=32)
 		.right(fourth),
 		.select(select[0]),
 		.out(upper));
-		
+
 	mux2 final_mux(
 		.left(lower),
 		.right(upper),
 		.select(select[1]),
 		.out(out));
-endmodule
+endmodule