Definition replace_externs (mu:SM_Injection) fSrc' fTgt': SM_Injection :=
match mu with
Build_SM_Injection locBSrc locBTgt pSrc pTgt local extBSrc extBTgt fSrc fTgt extern =>
Build_SM_Injection locBSrc locBTgt pSrc pTgt local extBSrc extBTgt fSrc' fTgt' extern
end.
(*typically, we have forall b, fSrc b -> fSrc' b and forall b, fTgt b -> fTgt' b, i.e. only reclassify unknown entries as foreign*)

Lemma replace_externs_frgnBlocksTgt: forall mu fSrc' fTgt',
frgnBlocksTgt (replace_externs mu fSrc' fTgt') = fTgt'.