sum1.test

<<< native
[ EUnderover
    False
    (ESymbol Op "\8721")
    (EGrouped [ EIdentifier "i" , ESymbol Rel "=" , ENumber "0" ])
    (EIdentifier "\8734")
, ESub (EIdentifier "x") (EIdentifier "i")
]
>>> tex
\sum\limits_{i = 0}^{\infty}x_{i}