2. The XAS directory tree
The XAS installation comes as a directory tree rooted under a logical position referred
to by environment variable $XASTOP. It shall not contain any file
(except for makefiles created by xasbuild or other compilation support files,
but only the subdirectories listed below.
This does not refer to the area where the data is stored, which is instead arranged
freely by each individual users according to the guidelines described elsewhere
The subdirectories are