  Charge-independence breaking due to the pion-mass difference in the (chiral)
two-pion-exchange nucleon-nucleon force is investigated. A general argument
based on symmetries is presented that relates the charge-symmetric part of that
force to the proton-proton case. The static potential linear in that mass
difference is worked out as an explicit example by means of Feynman diagrams,
and this confirms the general argument.
