"... we must explicitly include into the rest mass of a system also the
energy of all the interaction fields in terms of the field theory,
e.g., E(EM)= 1/2(E^2 + B^2) for the EM field (stupid constants dropped)."
- the last line should read E (EM)= the volume integral of 1/2(E^2 + B^2),
with E (EM) on the left being electromagnetic energy, and on the right -
electric field.