<<< native [ EUnderover False (EIdentifier "B") (EIdentifier "under") (EIdentifier "over") , ESymbol Rel "=" , EOver False (EUnder False (EIdentifier "B") (EIdentifier "under")) (EIdentifier "over") ] >>> mml $$\underset{\mathrm{under}}{\overset{\mathrm{over}}{B}}=\stackrel{\mathrm{over}}{\underset{\mathrm{under}}{B}}$$