diff -r 5deefcc8cffa -r e3acf2bf3895 progs/catastrophic.py
--- a/progs/catastrophic.py	Tue Sep 20 12:24:29 2016 +0100
+++ b/progs/catastrophic.py	Tue Sep 20 12:25:09 2016 +0100
@@ -2,11 +2,23 @@
 import re
 import sys
 
+# case of catastrophic backtracking in Python
+#
+# regex: (a?){n} a{n}
+# strings: aa...
+#
+# call with timing as:
+#
+#   > time ./catastrophic.py 20
+
+# counter n given on the command line
 cn = sys.argv[1]
 
+# constructing the regex
 r1 = '((a?){%s})' % cn
 r2 = 'a{%s}' % cn
 
+# calling the matching function
 m = re.match(r1 + r2 , "a" * int(cn)) 
 
 print m.group(0)