<<< native [ ESub (EIdentifier "P") (ENumber "0") , ESymbol Rel "=" , EUnderover False (ESub (EIdentifier "P") (ENumber "0")) (EPhantom (EIdentifier "x")) (EPhantom (EIdentifier "x")) ] >>> mml $$P_0=\underset{\phantom{x}}{\overset{\phantom{x}}{P_0}}$$