15.5.1.1 Pack requirements on Prolog

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