mathlib

93a15d6b - feat(ring_theory/subring): field structure on centers of a division_ring (#8472)

feat(ring_theory/subring): field structure on centers of a division_ring (#8472) I've also tidied up the subtitles. Previously there was a mix of h1 and h3s, I've made them all h2s. Co-authored-by: Eric Wieser <wieser.eric@gmail.com>