Adv550 original sources

The gzipped tarball orig550.tgz 114.6 KB, contains:

• Dave Platt's unmodified A-code source of Adv550, dated 18 Sep 1984.
• My reconstructions of Platt's original F77 munger and executive to handle the A-code source.
• A subdirectory of PL6 sources (as preserved in the IF Archive)
• A separate document explaining what's what, and saying a few words about this package came about.