<<< native
[ ESub (EIdentifier "P") (ENumber "0")
, ESymbol Rel "="
, EUnderover
    False
    (ESub (EIdentifier "P") (ENumber "0"))
    (EPhantom (EIdentifier "x"))
    (EPhantom (EIdentifier "x"))
]
>>> tex
P_{0} = \underset{\phantom{x}}{\overset{\phantom{x}}{P_{0}}}