texmath-0.12.5: test/roundtrip/msubsup-10.native

[ ESubsup (EIdentifier "p") (EIdentifier "x") (ENumber "3")
, ESymbol Rel "="
, EUnder
    False
    (ESubsup (EIdentifier "p") (EIdentifier "x") (ENumber "3"))
    (EPhantom (EText TextNormal "l"))
]