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add args.m example

--- a/Args/README.md

+++ b/Args/README.md

@@ -1,23 +1,29 @@

 # Command-Line Arguments

-Inferno has a dedicated module for processing commandline flags and arguments, arg(2). 

+Inferno has a dedicated module for processing commandline flags and arguments, [arg(2)](http://man.cat-v.org/inferno/2/arg). 

 ## Source

 ### args.b:17,22

+The module `Arg` is loaded and initialized. As per the manual, the `arg->init()` function must be called before any other functions can be called from `Arg`. 

+The usage message is also set pre-emptively for use later, if necessary.

 ### args.b:24,37

+There are two flag arguments, `r` and `c` which set reversal of arguments and the list item indicator mark, respectively.

+The flag `r` is a binary option toggle, the flag's presence as an argument is sufficient to change the value of `rev`. 

+The flag `c` takes an argument. The `arg->earg()` function is used to pop the flag's argument out of the list. Specifically with regards to `earg()` rather than `arg()` as per the manual, `earg()` will call `arg->usage()` if the relevant argument does not exist. 

+

 Note: After the processing of all flags, `argv` is re-set from the value contained within `arg` to remove the elements (if any) utilized by flags and their arguments. 

 ### args.b:41,51

+This section utilizes the `rev` variable to (naively) reverse the list `argv`. After the `rev` check, the list `argv` is printed in order from beginning to end with each element being printed on a new line with the `mark` variable being prefixed to the list element currently at the head of the `argv` list each iteration during printout. 

-

 ## Demo

 	; limbo args.b

@@ -48,3 +54,9 @@

 	->  c

 	->  d

 	; 

+

+## Exercises

+

+- Can you trick `earg()` into accepting a nil value?

+- How would you pull an `int` out of `earg()`?

+- How would you trip the `usage()` message call-able from `earg()`?

--- a/README.md

+++ b/README.md

@@ -47,6 +47,10 @@

 - [Generics, Picks, and Interfaces (kind of)](./Generics)

 - [Exceptions](./Exceptions)

+Standard library modules:

+

+- [Command-Line Arguments](./Args)

+

 ## References

 - [The Limbo Programming Language](http://doc.cat-v.org/inferno/4th_edition/limbo_language/limbo)