Cstring Literals
   Two features of string literal values come to the fore here.
   First, you can pass a string literal to a Cstring argument
   if that argument is const.  (See below for more.)

   Second, when the compiler finds two string literals separated
   by naught but whitespace, it will automatically concatenate
   them.  Note how this aids in the building of the fifth error
   message in the array (which would have normally been too long
   and been cut off at the edge of a printout).

Cstring Arguments
   By design, Cstring arguments require no data length argument
   accompany them to functions.  But, one often overlooked
   feature of Cstring arguments is their use in generalizing
   a function without losing centrality (lessening cohesion).

   By passing things such as prompts and error messages to your
   functions via Cstring arguments, your caller can make all user
   interaction just as they please.  Note how this is done in
   the get_nonneg() function.

   Even better, note how by having the Cstring arguments be
   const, the caller can provide their prompt and error message
   as literal strings.  Thus they can avoid having to come up
   silly names for variables to hold these Cstrings.

Two-D Arrays
   An array of Cstrings is by its nature 2D.  We can hide this
   aspect by using a typedef (type definition).  Such typedef's
   can be placed in a particular function, global to a program
   file, or in a library for anyone to #include and use.

   Please note that the typedef for an array (Cstring here) also
   needs a constant to go along with it.  This should be placed
   immediately before the typedef where-ever that may end up (see
   above).

   The Message typedef in this program is hidden inside the
   function as it isn't used anywhere else.  Note how the
   messages array appears 1D, but is in reality 2D because
   its base type is a 1D array.

Branching
   Yes, both if structures in this program could have been done
   as ?: operations.  However, for clarity, I chose to do them
   out the long way.  Please feel free to re-do them as ?:s as
   an exercise.  *smile*