<<< native
[ EUnderover
    False
    (EIdentifier "B")
    (EIdentifier "underscript")
    (EIdentifier "overscript")
, ESymbol Rel "="
, EOver
    False
    (EUnder
       False
       (EGrouped
          [ EPhantom (ENumber "1")
          , EIdentifier "B"
          , EPhantom (ENumber "1")
          ])
       (EIdentifier "underscript"))
    (EIdentifier "overscript")
]
>>> eqn
B from underscript to overscript = {{hphantom 1 B hphantom 1} from underscript} to overscript