// convenience method for callers that know only one sstable is involved in the transaction
public SSTableReader onlyOne() {
assert originals.size() == 1;
return getFirst(originals, null);
}
/** check this transaction has never been used */
private void checkUnused() {
assert logged.isEmpty();
assert staged.isEmpty();
assert identities.size() == originals.size();
assert originals.size() == marked.size();
}