diff --git a/homework4/starting_point/.idea/workspace.xml b/homework4/starting_point/.idea/workspace.xml
index fed343f..103da57 100644
--- a/homework4/starting_point/.idea/workspace.xml
+++ b/homework4/starting_point/.idea/workspace.xml
@@ -1,449 +1,432 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <project version="4">
   <component name="CMakeRunConfigurationManager" shouldGenerate="true" shouldDeleteObsolete="true" buildAllGenerated="true">
     <generated>
       <config projectName="Particles" targetName="test_kepler" />
       <config projectName="Particles" targetName="part" />
       <config projectName="Particles" targetName="test_fft" />
       <config projectName="Particles" targetName="particles" />
     </generated>
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   <component name="CMakeSettings">
     <configurations>
       <configuration PROFILE_NAME="Debug" CONFIG_NAME="Debug" />
     </configurations>
   </component>
   <component name="ChangeListManager">
-    <list default="true" id="b4e9d45a-71a7-4f96-94b1-4bcb4ac5e568" name="Default Changelist" comment="" />
+    <list default="true" id="b4e9d45a-71a7-4f96-94b1-4bcb4ac5e568" name="Default Changelist" comment="">
+      <change beforePath="$PROJECT_DIR$/py_part.cpp" beforeDir="false" afterPath="$PROJECT_DIR$/py_part.cpp" afterDir="false" />
+    </list>
     <ignored path="$PROJECT_DIR$/cmake-build-debug/" />
     <option name="EXCLUDED_CONVERTED_TO_IGNORED" value="true" />
     <option name="SHOW_DIALOG" value="false" />
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         </counts>
       </usages-collector>
       <usages-collector id="statistics.file.extensions.open">
         <counts>
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+          <entry key="cc" value="13" />
           <entry key="cpp" value="1" />
-          <entry key="hh" value="11" />
+          <entry key="hh" value="13" />
           <entry key="txt" value="1" />
         </counts>
       </usages-collector>
       <usages-collector id="statistics.file.types.open">
         <counts>
           <entry key="CMakeLists.txt" value="1" />
-          <entry key="ObjectiveC" value="23" />
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         </counts>
       </usages-collector>
       <usages-collector id="statistics.file.extensions.edit">
         <counts>
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       <usages-collector id="statistics.file.types.edit">
         <counts>
           <entry key="CMakeLists.txt" value="201" />
-          <entry key="ObjectiveC" value="176" />
+          <entry key="ObjectiveC" value="251" />
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             </state>
           </provider>
         </entry>
       </file>
-      <file pinned="false" current-in-tab="false">
+      <file pinned="false" current-in-tab="true">
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         <option name="com.jetbrains.cidr.execution.CidrBuildBeforeRunTaskProvider$BuildBeforeRunTask" enabled="true" />
       </method>
     </configuration>
     <configuration name="particles" type="CMakeRunConfiguration" factoryName="Application" PASS_PARENT_ENVS_2="true" PROJECT_NAME="Particles" TARGET_NAME="particles" CONFIG_NAME="Debug" RUN_TARGET_PROJECT_NAME="Particles" RUN_TARGET_NAME="particles">
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       <item itemvalue="Application.part" />
       <item itemvalue="Application.particles" />
       <item itemvalue="Google Test.test_fft" />
       <item itemvalue="Google Test.test_kepler" />
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   <component name="SvnConfiguration">
     <configuration />
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   <component name="TaskManager">
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       <changelist id="b4e9d45a-71a7-4f96-94b1-4bcb4ac5e568" name="Default Changelist" comment="" />
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       <updated>1545641844292</updated>
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     <servers />
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     <history-entry file="test_fft - 2018.12.24 at 10h 59m 02s.xml">
       <configuration name="test_fft" configurationId="CMakeGoogleTestRunConfigurationType" />
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     <history-entry file="test_fft - 2018.12.24 at 11h 03m 49s.xml">
       <configuration name="test_fft" configurationId="CMakeGoogleTestRunConfigurationType" />
     </history-entry>
   </component>
   <component name="TimeTrackingManager">
-    <option name="totallyTimeSpent" value="18633000" />
+    <option name="totallyTimeSpent" value="20049000" />
   </component>
   <component name="ToolWindowManager">
     <frame x="1" y="23" width="1269" height="716" extended-state="0" />
     <editor active="true" />
     <layout>
       <window_info id="Favorites" side_tool="true" />
       <window_info active="true" content_ui="combo" id="Project" order="0" visible="true" weight="0.24938875" />
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     <option name="version" value="1" />
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   <component name="VcsContentAnnotationSettings">
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   <component name="editorHistoryManager">
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         </state>
       </provider>
     </entry>
-    <entry file="file://$PROJECT_DIR$/material_points_factory.cc">
+    <entry file="file://$PROJECT_DIR$/csv_writer.cc">
       <provider selected="true" editor-type-id="text-editor">
-        <state relative-caret-position="98">
-          <caret line="29" lean-forward="true" selection-start-line="29" selection-end-line="29" selection-end-column="16" />
+        <state relative-caret-position="-51">
           <folding>
-            <element signature="e#0#37#0" expanded="true" />
+            <element signature="e#0#24#0" expanded="true" />
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         </state>
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     </entry>
-    <entry file="file://$PROJECT_DIR$/py_part.cpp">
+    <entry file="file://$PROJECT_DIR$/material_points_factory.cc">
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-        <state relative-caret-position="202">
-          <caret line="33" column="57" selection-start-line="33" selection-start-column="57" selection-end-line="33" selection-end-column="57" />
+        <state relative-caret-position="33">
+          <caret line="29" lean-forward="true" selection-start-line="29" selection-end-line="29" selection-end-column="16" />
           <folding>
-            <element signature="e#0#21#0" expanded="true" />
+            <element signature="e#0#37#0" expanded="true" />
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     </entry>
-    <entry file="file://$PROJECT_DIR$/csv_writer.cc">
+    <entry file="file://$PROJECT_DIR$/system_evolution.cc">
       <provider selected="true" editor-type-id="text-editor">
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+        </state>
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+    </entry>
+    <entry file="file://$PROJECT_DIR$/ping_pong_balls_factory.hh">
+      <provider selected="true" editor-type-id="text-editor">
+        <state relative-caret-position="58">
+          <caret line="9" column="6" lean-forward="true" selection-start-line="9" selection-start-column="6" selection-end-line="9" selection-end-column="26" />
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     </entry>
     <entry file="file://$PROJECT_DIR$/main.cc">
       <provider selected="true" editor-type-id="text-editor">
-        <state relative-caret-position="335">
+        <state relative-caret-position="321">
           <caret line="58" column="25" selection-start-line="58" selection-start-column="25" selection-end-line="58" selection-end-column="25" />
           <folding>
             <element signature="e#0#29#0" expanded="true" />
           </folding>
         </state>
       </provider>
     </entry>
+    <entry file="file://$PROJECT_DIR$/py_part.cpp">
+      <provider selected="true" editor-type-id="text-editor">
+        <state relative-caret-position="166">
+          <caret line="39" column="20" selection-start-line="39" selection-start-column="20" selection-end-line="39" selection-end-column="20" />
+          <folding>
+            <element signature="e#0#21#0" expanded="true" />
+          </folding>
+        </state>
+      </provider>
+    </entry>
   </component>
   <component name="masterDetails">
     <states>
       <state key="ScopeChooserConfigurable.UI">
         <settings>
           <splitter-proportions>
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 </project>
\ No newline at end of file
diff --git a/homework4/starting_point/py_part.cpp b/homework4/starting_point/py_part.cpp
index e49452f..274f081 100644
--- a/homework4/starting_point/py_part.cpp
+++ b/homework4/starting_point/py_part.cpp
@@ -1,41 +1,48 @@
 #include <functional>
 #include <iostream>
 #include <pybind11/pybind11.h>
 #include "particles_factory_interface.hh"
 #include "planets_factory.hh"
 #include "ping_pong_balls_factory.hh"
 #include "material_points_factory.hh"
 
 namespace py = pybind11;
 
 PYBIND11_MODULE(pypart, m) {
     m.doc() = "pybind particle plugin"; // optional docstring
 
     // wrap the class
     py::class_<ParticlesFactoryInterface>(m, "ParticlesFactoryInterface")
             .def("getInstance", &ParticlesFactoryInterface::getInstance,
                     py::return_value_policy::reference);
 
     py::class_<PingPongBallsFactory, ParticlesFactoryInterface>(m, "PingPongBallsFactory")
             .def("getInstance", &PingPongBallsFactory::getInstance,
                     py::return_value_policy::reference)
-            .def("createSimulation", &PingPongBallsFactory::createSimulation);
+            .def("createSimulation", &PingPongBallsFactory::createSimulation,
+                 py::return_value_policy::reference);
 
     py::class_<PlanetsFactory, ParticlesFactoryInterface>(m, "PlanetsFactory")
             .def("getInstance", &PlanetsFactory::getInstance,
                     py::return_value_policy::reference)
-            .def("createSimulation", &PlanetsFactory::createSimulation);
+            .def("createSimulation", &PlanetsFactory::createSimulation,
+                 py::return_value_policy::reference);
 
     py::class_<MaterialPointsFactory, ParticlesFactoryInterface>(m, "MaterialPointsFactory")
             .def("getInstance", &MaterialPointsFactory::getInstance,
                     py::return_value_policy::reference)
             .def("createSimulation",
                     (SystemEvolution & (MaterialPointsFactory::*)(const std::string &, Real))
-                    &MaterialPointsFactory::createSimulation
+                    &MaterialPointsFactory::createSimulation, py::return_value_policy::reference
             )
             .def("createSimulation", py::overload_cast<const std::string&, Real, py::function>
-                    (&MaterialPointsFactory::createSimulation<py::function>));
+                    (&MaterialPointsFactory::createSimulation<py::function>),
+                    py::return_value_policy::reference);
 
+    py::class_<SystemEvolution>(m, "SystemEvolution")
+            .def("setNSteps", &SystemEvolution::setNSteps)
+            .def("setDumpFreq", &SystemEvolution::setDumpFreq)
+            .def("evolve", &SystemEvolution::evolve);
 }