The file pack.pl may contain requires(Requirement)
statements. Normally, Requirement is a pack or token,
optionally with a version requirement. The requirement prolog
is reserved for requirements on the Prolog version while
prolog:Feature may be used to demand specific
features. Feature matching is described with
require_prolog_version/2.
Multiple requirements on Prolog must all be true. Below are some
examples
requires(prolog >= '9.2'). % 9.2.0 or later
requires(prolog:threads). % flag threads = true
requires(prolog:library(socket)). % library(socket) exists
requires(prolog:bounded(false)). % flag bounded = false
