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