<<< tex
a \choose {b \brace c + 2}

>>> native
[ EDelimited
    "("
    ")"
    [ Right
        (EFraction
           NoLineFrac
           (EIdentifier "a")
           (EDelimited
              "{"
              "}"
              [ Right
                  (EFraction
                     NoLineFrac
                     (EIdentifier "b")
                     (EGrouped [ EIdentifier "c" , ESymbol Bin "+" , ENumber "2" ]))
              ]))
    ]
]