<<< native
[ ESub (EIdentifier "L") (ESub (EIdentifier "X") (EIdentifier "h"))
, ESymbol Rel "="
, ESub
    (EIdentifier "L")
    (ESub
       (EIdentifier "X")
       (ESub (EIdentifier "h") (EPhantom (EIdentifier "fail"))))
]
>>> eqn
L sub {X sub h} = L sub {X sub {h sub {hphantom fail}}}