[m-rev.] for post-commit review: document exception condition for varset.new_vars/4

[Julien Fischer]

For post-commit review.

Document exception condition for varset.new_vars/4.

library/varset.m:
     Document that new_vars/4 will throw an exception if asked for
     a negative number of new variables.

     Fix a typo.

Julien.

diff --git a/library/varset.m b/library/varset.m
index fcb871a..a90f390 100644
--- a/library/varset.m
+++ b/library/varset.m
@@ -78,6 +78,8 @@
      varset(T)::in, varset(T)::out) is det.

      % Create multiple new variables.
+    % Throws an exception if a negative number of new variables
+    % is requested.
      %
  :- pred new_vars(int::in, list(var(T))::out,
      varset(T)::in, varset(T)::out) is det.
@@ -97,7 +99,7 @@

      % Delete the names and values for a sorted list of variables.
      % (If the list is not sorted, the predicate or function will
-    % either throw an exception of return incorrect output.)
+    % either throw an exception or return incorrect output.)
      %
  :- func delete_sorted_vars(varset(T), list(var(T))) = varset(T).
  :- pred delete_sorted_vars(list(var(T))::in,