<<< native
[ EUnderover
    True
    (ESymbol Op "\8721")
    (EGrouped [ EIdentifier "m" , ESymbol Rel "=" , ENumber "1" ])
    (ESymbol Ord "\8734")
, EUnderover
    True
    (ESymbol Op "\8721")
    (EGrouped [ EIdentifier "n" , ESymbol Rel "=" , ENumber "1" ])
    (ESymbol Ord "\8734")
, EFraction
    NormalFrac
    (EGrouped
       [ ESuper (EIdentifier "m") (ENumber "2")
       , ESpace (1 % 6)
       , EIdentifier "n"
       ])
    (EGrouped
       [ ESuper (ENumber "3") (EIdentifier "m")
       , EDelimited
           "("
           ")"
           [ Right (EIdentifier "m")
           , Right (ESpace (1 % 6))
           , Right (ESuper (ENumber "3") (EIdentifier "n"))
           , Right (ESymbol Bin "+")
           , Right (EIdentifier "n")
           , Right (ESpace (1 % 6))
           , Right (ESuper (ENumber "3") (EIdentifier "m"))
           ]
       ])
]
>>> tex
\sum_{m = 1}^{\infty}\sum_{n = 1}^{\infty}\frac{m^{2}\, n}{3^{m}\left( m\, 3^{n} + n\, 3^{m} \right)}