Definition nextIndex_preserved st st' :=
    (type st' = Leader ->
     type st = Leader /\
     maxIndex (log st) <= maxIndex (log st') /\
     nextIndex st' = nextIndex st).

Theorem handleClientRequest_nextIndex_preserved : forall h st client id c out st' ps, handleClientRequest h st client id c = (out, st', ps) -> nextIndex_preserved st st'.