minor change in rhs to improve performance

Author: elswit

src/+otp/+arenstorf/f.m

@@ -3,6 +3,6 @@
     D1= ((y(1)+m1)^2+y(2)^2)^(3/2);
     D2= ((y(1)-m2)^2+y(2)^2)^(3/2);
 
-    dy = [y(3);y(4);y(1)+2*y(4)-m2/D1*(y(1)+m1)-m1/D2*(y(1)-m2);y(2)-2*y(3)-m2/D1*y(2)-m1/D2*y(2)];
+    dy = [y(3:4);y(1)+2*y(4)-m2/D1*(y(1)+m1)-m1/D2*(y(1)-m2);y(2)-2*y(3)-m2/D1*y(2)-m1/D2*y(2)];
 
 end