diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..637f261
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1 @@
+uml_diagram.tex
diff --git a/include/World.h b/include/World.h
index cdb2b76..b9b94df 100644
--- a/include/World.h
+++ b/include/World.h
@@ -14,7 +14,7 @@ class World {
     int getSurfaceLevel ( int x, int y ) const;
     int mine ( int x, int y );
 
-    void display ( int p1x = -1, int p1y = -1, int p2x = -1, int p2y = -1 ) const;
+    void display ( int p1x = -1, int p1y = -1, int p2x = -1, int p2y = -1 ) const; // player positions initialized to -1 -> outside of surface grid
 
     int getSizeX () const { return _sizeX; }
     int getSizeY () const { return _sizeY; }
diff --git a/src/World.cpp b/src/World.cpp
index 352e770..975881d 100644
--- a/src/World.cpp
+++ b/src/World.cpp
@@ -4,7 +4,6 @@
 #include <random>
 
 #include <algorithm>
-#include <atomic>
 
 World::World ( int x, int y, int z )
     : _sizeX ( x ), _sizeY ( y ), _sizeZ ( z ),
@@ -94,7 +93,7 @@ void World::display ( int p1x, int p1y, int p2x, int p2y ) const {
                 std::cout << std::setw ( 4 ) << val;
         }
         std::cout << "\n";
-        std::cout << "=================================\n";
+        std::cout << "=========================\n";
     }
 }
 
diff --git a/uml_diagram.pdf b/uml_diagram.pdf
new file mode 100644
index 0000000..038ede1
Binary files /dev/null and b/uml_diagram.pdf differ