IndexAtLocation(AtomIndex, SubIndexType, prevSubIndex) : null;
_ => SubIndex?.Previous is MathListIndex prevSubIndex?IndexAtLocation(AtomIndex, SubIndexType, prevSubIndex) : null,
SubIndex?.LevelDown() is MathListIndex subIndex?IndexAtLocation(AtomIndex, SubIndexType, subIndex) :
_ => IndexAtLocation(AtomIndex, SubIndexType, SubIndex?.Next),