[m-rev.] diff: add string.is_empty/1

[Julien Fischer]

Add string.is_empty/1.

library/string.m:
         Add the test predicate is_empty/1 for strings.
         This is useful when working with higher-order code.

NEWS:
         Announce the above addition.

         Unrelated change:
 	s/terminating newline character/terminating newline/
         since a newline may be composed of multiple characters.

Julien.

diff --git a/NEWS b/NEWS
index 9e29c31..d3e35d8 100644
--- a/NEWS
+++ b/NEWS
@@ -32,7 +32,7 @@ Changes to the Mercury standard library:

  * We have added the print_line and write_line family of predicates to the
    io module.  These behave like the print and write predicates, but also
-  write a terminating newline character.
+  write a terminating newline.

  * io.print and string_writer.print now print arbitrary precision integers
    in their decimal form instead of printing their underlying representation.
@@ -105,9 +105,10 @@ Changes to the Mercury standard library:
     - error/2
     - func_error/2

-* The following predicate has been added to the string module:
+* The following predicates have been added to the string module:

     - is_all_alnum/1
+   - is_empty/1

  * The following predicates have been added to the map module:

diff --git a/library/string.m b/library/string.m
index 8a57d7b..9283b2b 100644
--- a/library/string.m
+++ b/library/string.m
@@ -396,6 +396,9 @@
  %
  % Tests on strings.
  %
+    % True if string is the empty string.
+    %
+:- pred is_empty(string::in) is semidet.

      % True if string contains only alphabetic characters [A-Za-z].
      %
@@ -2746,6 +2749,8 @@ string.hash6_loop(String, Index, Length, !HashVal) :-
  % For speed, most of these predicates have C versions as well as
  % Mercury versions. XXX why not all?

+is_empty("").
+
  :- pragma foreign_proc("C",
      is_all_alpha(S::in),
      [will_not_call_mercury, promise_pure, thread_safe, will_not_modify_trail,