substack.test

<<< tex
\sum_{\substack{
   0<i<m \\
   0<j<n
  }}
 P(i,j)

>>> native
[ EUnder
    True
    (ESymbol Op "\8721")
    (EArray
       [ AlignCenter ]
       [ [ [ EGrouped
               [ ENumber "0"
               , ESymbol Rel "<"
               , EIdentifier "i"
               , ESymbol Rel "<"
               , EIdentifier "m"
               ]
           ]
         ]
       , [ [ EGrouped
               [ ENumber "0"
               , ESymbol Rel "<"
               , EIdentifier "j"
               , ESymbol Rel "<"
               , EIdentifier "n"
               ]
           ]
         ]
       ])
, EIdentifier "P"
, EDelimited
    "("
    ")"
    [ Right (EIdentifier "i")
    , Right (ESymbol Pun ",")
    , Right (EIdentifier "j")
    ]
]