Relation between energy conservation and equations of motion (a reply to the comment by E A Arinshtein [ Phys. Usp. 58 309 (2015); Usp. Fiz. Nauk 185 333 (2015)] on “Analytical mechanics and field theory: derivation of equations from energy conservation” [ Phys. Usp. 57 593 (2014); Usp. Fiz. Nauk 184 641 (2014)])