Add explanatory text for the subcommand example

SC2145.md

@@ -24,25 +24,30 @@ If the intention is to provide each array element as a separate argument, put th
 
 ### Exceptions
 
+Concatenating a string with an array can be used to make a command line interface with subcommands.
+
+To implement the subcommand interface, first pick a prefix like `subcommand_` below for the function names that implement the subcommands.  Then protect the parsing of the subcommands by listing them in the patterns of a case statement.  In the body of the case statement, a concatenation of the prefix with `"$@"` will invoke the subcommand function and pass the rest of the parameters to the function.
+
+For example:
 ```sh
-command_foo() {
+subcommand_foo() {
   echo "In foo"
   echo "\$1 == $1 == aaa"
   echo "\$2 == $2 == bbb"
 }
 
-command_bar() {
+subcommand_bar() {
   echo "In bar"
 }
 
-command_baz() {
+subcommand_baz() {
   echo "In baz"
 }
 
 main() {
   case "$1" in
     foo | bar | baz )
-      command_"$@"
+      subcommand_"$@"
     ;;
     * )
       printf "Error: %s\n" "Unrecognized command"
@@ -52,3 +57,5 @@ main() {
 
 main foo aaa bbb
 ```
+
+In the above example, inside the `main` function, the value of `"$@"` is `( foo aaa bbb ccc )`.  The value of `subcommand_"$@"` after expanding is `( subcommand_foo aaa bbb ccc )`, which the shell interprets as a call to the `subcommand_foo` function.