msub-06.test

<<< native
[ ESub (EIdentifier "U") (EIdentifier "t")
, ESymbol Rel "="
, ESub (EIdentifier "U") (EIdentifier "t")
, EPhantom
    (EFraction NormalFrac (EIdentifier "test") (EIdentifier "failed"))
]
>>> tex
U_{t} = U_{t}\phantom{\frac{test}{failed}}