Directory: double

These files illustrate inheritance by using tagged types as well as
child packages. A generic list package defines type pointer as access to
the class of listnode. Listnode is a tagged type, which is extended with
a "previous" pointer to provide type dlistnode, a node in a doubly-linked 
list.

gnat version 2.06 for DOS had trouble with the visibility rules for child 
packages, so I could not compile these files under it. I was able to compile
them successfully with version 3.0 at TRI-Ada, and after getting GNAT version
3.00w (Sparc-Sun-Solaris) I compiled them locally. Note that the files had
to use different names to run under Solaris, but I listed them here with 
their DOS names as I expect they will be compilable under version 3.0 of 
GNAT for DOS when it becomes available.

 I can't "with" both listn.io and listn.dbl.io in the same test 
program, since the linker has problems with it. The test program is made
to work by using only the io package for singly-linked lists, since doubly-
linked lists are printed in the same way.  

The files include:

listn.ads	define a generic (by element type) list package,
listn.adb	providing constructors, selectors, testers, and mutators

int_io.ads	an instantiation of text_io.ineger_io for type Integer

intlistn.ads	instantiates listn for integer elements

listn-io.ads	define a generic child package of the list package that
listn-io.adb	requires an additional parameter, a Put on the element type,
		and defines a Put on lists.

intlisio.ads	instantiates the io child for integer lists

listndbl.ads	define a generic child package of the list package that
listndbl.adb	extends the listnode to include a previous pointer, creating
		doubly-linked lists

intlidbl.ads	instantiates the doubly-linked list package for integers

lisdblio.ads	define a generic child package of the doubly-linked list
lisdblio.adb	package, taking an additional generic parameter, a Put on the
		element type, and providing a Put for the doubly-linked lists
		of that element type. 

intdblio.ads	instantiates the doubly-linked io package for integers. 
		(This is where the older version of gnat complains about 
		listn.dbl.io. It claims that Pointer, IsEmpty, First, and 
		Rest are not directly visible. These are all defined in the 
		grandparent generic package listn. listn is instantiated to 
		intlistn, listn.dbl is instantiated to intlistn.dbl (in 
		intlidbl.ads), then this file instantiates listn.dbl.io to 
		intlistn.dbl.io. )

chkdbl.adb	tests packages above by creating, modifying, and printing 
		both singly and doubly-linked lists