<<< native
[ EUnderover
    False (EIdentifier "BASE") (EIdentifier "s") (EIdentifier "t")
, ESymbol Rel "="
, EUnderover
    False
    (EIdentifier "BASE")
    (EGrouped
       [ EPhantom (ENumber "1")
       , EIdentifier "s"
       , EPhantom (ENumber "1")
       ])
    (EGrouped
       [ EPhantom (ENumber "1")
       , EIdentifier "t"
       , EPhantom (ENumber "1")
       ])
]
>>> tex
\underset{s}{\overset{t}{BASE}} = \underset{\phantom{1}s\phantom{1}}{\overset{\phantom{1}t\phantom{1}}{BASE}}