This reference variable holds the directory (folder) for the base of the HOL
Light distribution. This information is used, for example, when loading files
with loads. Normally set to the current directory when HOL Light is loaded or
built, but picked up from the system variable HOLLIGHT_DIR if it is defined.
On my laptop, the value is:
val it : string = "/home/johnh/holl"
Ensuring that HOL Light can find any libraries or other system files needed to