diff --git a/data/example-snaphot.xsgp b/data/example-snaphot.xsgp
new file mode 100644
index 0000000..a9359a0
--- /dev/null
+++ b/data/example-snaphot.xsgp
@@ -0,0 +1,33 @@
+<?xml version="1.0"?>
+<!DOCTYPE sgpem SYSTEM "sgpem.dtd">
+<sgpem>
+  <resources>
+    <resource name="Resource 1" id="reskey2" arrival-time="0" how-many="1" pre-emptible="false"/>
+    <resource name="Invalid? Resource &lt;n&gt; 1" id="reskey3" arrival-time="0" how-many="1" pre-emptible="false"/>
+  </resources>
+  <schedulables>
+    <process name="Process 1" priority="2" arrival-time="0">
+      <threads>
+        <thread name="Process 1 - Thread 1" priority="6" arrival-delta="2" lasts-for="0">
+          <requests>
+            <request arrival-time="3">
+              <subrequest resource="2" how-many="2" lasts-for="5"/>
+            </request>
+          </requests>
+        </thread>
+        <thread name="Process 1 - Thread 2" priority="5" arrival-delta="3" lasts-for="0">
+          <requests/>
+        </thread>
+      </threads>
+    </process>
+    <process name="Process 2" priority="3" arrival-time="7">
+      <threads/>
+    </process>
+    <process name="Process 3" priority="1" arrival-time="9">
+      <threads/>
+    </process>
+    <process name="Invalid? &lt;process/&gt; &amp;3 or\4" priority="1" arrival-time="9">
+      <threads/>
+    </process>
+  </schedulables>
+</sgpem>
diff --git a/data/sgpem.dtd b/data/sgpem.dtd
new file mode 100644
index 0000000..db309b9
--- /dev/null
+++ b/data/sgpem.dtd
@@ -0,0 +1,38 @@
+<!ELEMENT sgpem (resources, schedulables)>
+
+<!ENTITY % text "#PCDATA">
+
+
+<!ELEMENT schedulables (process)*>
+<!ELEMENT process (threads)>
+<!ATTLIST process 
+    name CDATA #IMPLIED
+    priority CDATA #IMPLIED
+    arrival-time CDATA #REQUIRED >
+<!-- <!ELEMENT threads (thread)+> -->
+<!ELEMENT threads (thread)*>
+<!ELEMENT thread (requests)>
+<!ATTLIST thread 
+    name CDATA #IMPLIED
+    priority CDATA #IMPLIED
+    arrival-delta CDATA #IMPLIED
+    lasts-for CDATA #REQUIRED >
+
+<!ELEMENT resources (resource)*>
+<!ELEMENT resource EMPTY>
+<!ATTLIST resource 
+    name CDATA #IMPLIED
+    id ID #REQUIRED
+    arrival-time CDATA #REQUIRED
+    how-many CDATA #IMPLIED
+    pre-emptible (true | false) "false" >
+
+<!ELEMENT requests (request)*>
+<!ELEMENT request (subrequest)+>
+<!ATTLIST request
+    arrival-time CDATA #REQUIRED >
+<!ELEMENT subrequest EMPTY>
+<!ATTLIST subrequest
+    resource IDREF #REQUIRED
+    how-many CDATA #IMPLIED
+    lasts-for CDATA #REQUIRED >