<<< 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")
    ]
]
>>> eqn
sum from {matrix{
ccol{ {0 < i < m} above {0 < j < n} }
}} P left ( i , j right )