mention backtrackable and non-backtrackable global variables

diff --git a/README.md b/README.md
index e30bd1b7de39402bde77667a3d6b988f8c8d86b0..a25a560d5d4da3c41121b53da9c8f584c1e0593d 100644 (file)
--- a/README.md
+++ b/README.md
@@ -281,12 +281,15 @@ Logic Programming (LP) and Constraints.
 
 In addition to built-in support for [`dif/2`](src/prolog/lib/dif.pl),
 [`freeze/2`](src/prolog/lib/freeze.pl),
-[CLP(ℤ)](src/prolog/lib/clpz.pl) and [CLP(B)](src/prolog/lib/clpb.pl),
+[CLP(B)](src/prolog/lib/clpb.pl) and [CLP(ℤ)](src/prolog/lib/clpz.pl),
 Scryer provides a convenient way to implement new user-defined
 constraints: *Attributed variables* are available via
 [`library(atts)`](src/prolog/lib/atts.pl) as in SICStus Prolog,
 which is one of the most sophisticated and fastest constraint systems
-in existence.
+in existence. In [`library(iso_ext)`](src/prolog/lib/iso_ext.pl),
+Scryer provides predicates for backtrackable (`bb_b_put/2`) and
+non-backtrackable (`bb_put/2`) global variables, which are needed to
+implement certain types of constraint solvers.
 
 These features make Scryer Prolog an ideal platform for teaching,
 learning and developing portable CLP applications.