This library is provided to make it easier to reason about software versions, in particular require that that hosting Prolog system is of the right version and provides the features required by the library or application.
Major.Minor[[.Patch][[-GitRev],-GitHash]]]
Example strings are’8.5’,’8.5.0’,’8.5.0-50’,’8.5.0-69-gad38e8ad8`. The last two require fetching the sources from git or using the Windows daily builds.
Versions may be embedded in a comparison operator (<, =<, =,
>= or >), e.g., =<('9.1'). 
Versions are considered to compare equal only on the components of the Required 
version. I.e., '9.1' compares equal to '9.1.2'.
Version expressions can be constructed from the Prolog operators’,’/2,’;’/2 
and’\+’/1. An example of a complicated 
expression is below, which demands major version 9, but considers 9.1.2 
not suitable.
(>=('9'), \+(=('9.1.2')))
Features is a list of required or preferred features. Individual features are:
library(Lib) to be present. Thde library not being 
there may indicate an incomplete installation. For example
library(pce) to demand xpce graphics support.
current_prolog_flag(Flag, true) to be true.
current_prolog_flag(Flag, Value) 
to be true.
<, =<, =, >= 
or >. If
Cmp is unbound we check whether < or > 
hold or else bind Cmp to
=.
When comparing for equality (=), the versions are 
considered equal if they compare equal up to the detail level of the 
least specified. E.g,’9.1.2’is considered equal to’9.1’.