<<< native
[ ESub (EIdentifier "A") (ESub (EIdentifier "B") (EIdentifier "C"))
, ESymbol Rel "="
, ESub
    (EIdentifier "A")
    (ESub
       (EIdentifier "B")
       (EUnder False (EIdentifier "C") (EPhantom (EIdentifier "D"))))
]
>>> eqn
A sub {B sub C} = A sub {B sub {C from {hphantom D}}}