<<< 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"))))
]
>>> eqn
M sub {B sub 4 sup 3} sup {T sub 2 sup 1} = M sub {B sub {4 from {hphantom {roman "0"}}} sup 3} sup {T sub 2 sup {1 to {hphantom {roman "0"}}}}