adding methods to position

lib/position.dart

@@ -1,7 +1,25 @@
+import 'dart:math' as math;
+
 class Position {
   double x, y;
 
   Position(this.x, this.y);
 
   Position.empty() : this(0.0, 0.0);
+
+  Position add(Position other) {
+    this.x += other.x;
+    this.y += other.y;
+    return this;
+  }
+
+  Position opposite() {
+    this.x *= -1;
+    this.y *= -1;
+    return this;
+  }
+
+  double length() {
+    return math.sqrt(math.pow(this.x, 2) + math.pow(this.y, 2));
+  }
 }