<<< native
[ ESuper (EIdentifier "X") (ENumber "2")
, ESymbol Rel "="
, EIdentifier "y"
, EGrouped
    [ EText TextNormal " under "
    , ESub (EIdentifier "H") (ENumber "0")
    , EText TextNormal " except when "
    , EGrouped
        [ EIdentifier "x"
        , EGrouped
            [ EText TextBold " is less than "
            , EIdentifier "z"
            , EText TextBold "."
            ]
        ]
    ]
]
>>> eqn
X sup 2 = y {roman " under " H sub 0 roman " except when " {x {bold " is less than " z bold "."}}}