// assumes concepts have been written out
static File backupConceptsNow(Concepts cc default db_mainConcepts()) {
  if (cc == null) null;
  File f1 = cc.conceptsFile();
  File f2 = fileInSameDir(f1, "backups/concepts.structure" + (cc.useGZIP ? ".gz" : "") + ".backup" + ymd_minus_hms());
  ret copyFile(f1, f2);
}