                            ASIS/Program View Layer

                               common Subsystem
                               

This subsystem contains utility operations and data structure abstractions
that are shared by the view subsystems.

Diagnostic messages generated by the view subsystems are written to a message 
log abstraction using the package Msg_Log.  The implementation of Msg_Log
writes message text to the Standard Error device using Text_IO.  The package
Text_IO_Supplement constructs a Text_IO file object for Standard Error; 
THE CURRENT IMPLEMENTATION IS SPECIFIC TO RATIONAL APEX!  IT MAY HAVE TO BE 
CHANGED FOR OTHER ADA ENVIRONMENTS.  (For example, Verdix VADS includes
a Standard_Error function in its Text_IO package; in this case, the function
Text_IO_Supplement.Standard_Error can simply call Text_IO.Standard_Error.)

The data structure abstractions are from Booch, "Software Components with Ada",
Benjamin-Cummings, 1987.
