<<< native
[ ESubsup
    (EIdentifier "M")
    (ESubsup (EIdentifier "B") (ENumber "4") (ENumber "3"))
    (ESubsup (EIdentifier "T") (ENumber "2") (ENumber "1"))
, ESymbol Rel "="
, ESubsup
    (EIdentifier "M")
    (ESubsup
       (EIdentifier "B")
       (EUnder False (ENumber "4") (EPhantom (EText TextNormal "0")))
       (ENumber "3"))
    (ESubsup
       (EIdentifier "T")
       (ENumber "2")
       (EOver False (ENumber "1") (EPhantom (EText TextNormal "0"))))
]
>>> tex
M_{B_{4}^{3}}^{T_{2}^{1}} = M_{B_{\underset{\phantom{\text{0}}}{4}}^{3}}^{T_{2}^{\overset{\phantom{\text{0}}}{1}}}