  The equation derived by F.Rohrlich (Phys.Rev.E 77, 046609 (2008)) reproduces
Eq.(76.3) from the Landau and Lifshitz book (The Classical Theory of Fields).
The new validity condition for this equation is inapplicable as it is, but once
fixed it coincides with Eqs.(75.11-12) in the cited book.
