Add Pop to the list of idents that GetNextShortest can't generate

It's unlikely to hit this one (requires more than 10,000 identifiers to trigger) but it's a bug nevertheless.

While touching the file, try to explain the situation better in the first comment.

lslopt/lslrenamer.py

@@ -15,11 +15,12 @@
 #    You should have received a copy of the GNU General Public License
 #    along with LSL PyOptimizer. If not, see <http://www.gnu.org/licenses/>.
 
-# This module renames all kinds of variables. Globals and function/event
-# parameters take memory space, so shrinking the identifiers as much as
-# possible ensures their memory usage will be minimized. It also reuses some
-# preexisting names when possible. Locals are renamed also so that they don't
-# stand in the way of globals.
+# This module renames all kinds of variables. Global variables, user functions,
+# state names, event names and function/event parameters all take memory space,
+# so shrinking the identifiers as much as possible, when possible (i.e. not for
+# events or the default state), ensures their memory usage will be minimized.
+# It also reuses some preexisting names. Locals are renamed also so that they
+# don't stand in the way of globals.
 #
 # A side effect of this change is that the script becomes unreadable gibberish.
 
@@ -28,8 +29,11 @@ from strutil import xrange
 class renamer(object):
     CharSet1 = '_ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz'
     CharSet2 = '0123456789_ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz'
+    # As a special case, 'Pop' is not a keyword, but it's one of the predefined
+    # identifiers, therefore it must not be generated by GetNextShortest
+    # because it's already in the list of identifiers available from the start.
     Kws = frozenset({'do', 'if', 'PI',
-                     'for', 'key', 'EOF',
+                     'for', 'key', 'EOF', 'Pop',
                      'jump', 'else', 'list', 'TRUE', 'LOOP', 'case'
                      })
     def GetNextShortest(self):