feat(data/real/ennreal): add ennreal lemma for `a / 3 + a / 3 + a / 3 = a`  (#11378)
feat(data/real/ennreal): add ennreal lemma for `a / 3 + a / 3 + a / 3 = a`  (#11378)






leanprover-community/mathlib,leanprover-community/mathlib