diff --git a/ssim/des.h b/ssim/des.h
index 006368d4b6..4e0c0b18e9 100644
--- a/ssim/des.h
+++ b/ssim/des.h
@@ -1,8 +1,32 @@
+// This file is part of BOINC.
+// http://boinc.berkeley.edu
+// Copyright (C) 2011 University of California
+//
+// BOINC is free software; you can redistribute it and/or modify it
+// under the terms of the GNU Lesser General Public License
+// as published by the Free Software Foundation,
+// either version 3 of the License, or (at your option) any later version.
+//
+// BOINC is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+// See the GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with BOINC.  If not, see <http://www.gnu.org/licenses/>.
+
+// The world's smallest discrete event simulator.
+// Uses the STL "heap" data structure for efficient event storage.
+
 #include <vector>
 #include <algorithm>
 
 using std::vector;
 
+// base class for events.
+// t is the time when the event occurs.
+// handle() is what you want to happen then
+//
 struct EVENT {
     double t;
     virtual void handle(){}
@@ -15,11 +39,17 @@ bool compare(EVENT* e1, EVENT* e2) {
 struct SIMULATOR {
     vector<EVENT*> events;
     double now;
+
+    // add an event
+    //
     void insert(EVENT* e) {
         //printf("adding %x\n", e);
         events.push_back(e);
         push_heap(events.begin(), events.end(), compare);
     }
+
+    // remove an event
+    //
     void remove(EVENT* e) {
         vector<EVENT*>::iterator i;
         //printf("removing %x\n", e);
@@ -33,6 +63,9 @@ struct SIMULATOR {
         }
         //printf("%x not found\n", e);
     }
+
+    // run the simulator for the given time period
+    //
     void simulate(double dur) {
         while (events.size()) {
             EVENT* e = events.front();