double rLimitMtime = 0.0;
    if( ridBackTo ){
      rLimitMtime = db_double(0.0,
         "SELECT mtime FROM event WHERE objid=%d",
         ridBackTo);
    }
    db_multi_exec(
      "WITH RECURSIVE "
      "  parent(pid,cid,isCP) AS ("
      "    SELECT plink.pid, plink.cid, 0 AS xisCP FROM plink"
      "    UNION ALL"
      "    SELECT parentid, childid, 1 FROM cherrypick WHERE NOT isExclude"
      "  ),"
      "  ancestor(rid, mtime, isCP) AS ("
      "    SELECT %d, mtime, 0 FROM event WHERE objid=%d "
      "    UNION "
      "    SELECT parent.pid, event.mtime, parent.isCP"
      "      FROM ancestor, parent, event"
      "     WHERE parent.cid=ancestor.rid"
      "       AND event.objid=parent.pid"
      "       AND NOT ancestor.isCP"
      "       AND (event.mtime>=%.17g OR parent.pid=%d)"
      "     ORDER BY mtime DESC LIMIT %d"
      "  )"
      "INSERT OR IGNORE INTO ok"
      "WITH RECURSIVE\n"
      "  parent(pid,cid,isCP) AS (\n"
      "    SELECT plink.pid, plink.cid, 0 AS xisCP FROM plink\n"
      "    UNION ALL\n"
      "    SELECT parentid, childid, 1 FROM cherrypick WHERE NOT isExclude\n"
      "  ),\n"
      "  ancestor(rid, mtime, isCP) AS (\n"
      "    SELECT %d, mtime, 0 FROM event WHERE objid=%d\n"
      "    UNION\n"
      "    SELECT parent.pid, event.mtime, parent.isCP\n"
      "      FROM ancestor, parent, event\n"
      "     WHERE parent.cid=ancestor.rid\n"
      "       AND event.objid=parent.pid\n"
      "       AND NOT ancestor.isCP\n"
      "       AND (event.mtime>=%.17g OR parent.pid=%d)\n"
      "     ORDER BY mtime DESC LIMIT %d\n"
      "  )\n"
      "INSERT OR IGNORE INTO ok SELECT rid FROM ancestor;",
      "  SELECT rid FROM ancestor;",
      rid, rid, rLimitMtime, ridBackTo, N
    );
    if( ridBackTo && db_changes()>1 ){
      db_multi_exec("INSERT OR IGNORE INTO ok VALUES(%d)", ridBackTo);
    }
  }
}

void compute_descendants(int rid, int N){
  if( !N ){
     N = -1;
  }else if( N<0 ){
     N = -N;
  }
  db_multi_exec(
    "WITH RECURSIVE"
    "  dx(rid,mtime) AS ("
    "     SELECT %d, 0"
    "     UNION"
    "     SELECT plink.cid, plink.mtime FROM dx, plink"
    "      WHERE plink.pid=dx.rid"
    "      ORDER BY 2"
    "  )"
    "WITH RECURSIVE\n"
    "  dx(rid,mtime) AS (\n"
    "     SELECT %d, 0\n"
    "     UNION\n"
    "     SELECT plink.cid, plink.mtime FROM dx, plink\n"
    "      WHERE plink.pid=dx.rid\n"
    "      ORDER BY 2\n"
    "  )\n"
    "INSERT OR IGNORE INTO ok SELECT rid FROM dx LIMIT %d",
    rid, N
  );
}

/*
** COMMAND: descendants*

#if INTERFACE
/* Flag parameters to compute_uses_file() */
#define USESFILE_DELETE   0x01  /* Include the check-ins where file deleted */

#endif

/*
** Append a new VALUES term.
*/
static void uses_file_append_term(Blob *pSql, int *pnCnt, int rid){
  if( *pnCnt==0 ){
    blob_append_sql(pSql, "(%d)", rid);
    *pnCnt = 4;
  }else if( (*pnCnt)%10==9 ){
    blob_append_sql(pSql, ",\n  (%d)", rid);
  }else{
    blob_append_sql(pSql, ",(%d)", rid);
  }
  ++*pnCnt;
}


/*
** Add to table zTab the record ID (rid) of every check-in that contains
** the file fid.
*/
void compute_uses_file(const char *zTab, int fid, int usesFlags){
  Bag seen;
  Bag pending;
  Stmt ins;
  Blob ins = BLOB_INITIALIZER;
  int nIns = 0;
  Stmt q;
  int rid;

  bag_init(&seen);
  bag_init(&pending);
  db_prepare(&ins, "INSERT OR IGNORE INTO \"%w\" VALUES(:rid)", zTab);
  blob_append_sql(&ins, "INSERT OR IGNORE INTO \"%w\" VALUES", zTab);
  db_prepare(&q, "SELECT mid FROM mlink WHERE fid=%d", fid);
  while( db_step(&q)==SQLITE_ROW ){
    int mid = db_column_int(&q, 0);
    bag_insert(&pending, mid);
    bag_insert(&seen, mid);
    db_bind_int(&ins, ":rid", mid);
    uses_file_append_term(&ins, &nIns, mid);
    db_step(&ins);
    db_reset(&ins);
  }
  db_finalize(&q);

  db_prepare(&q, "SELECT mid FROM mlink WHERE pid=%d", fid);
  while( db_step(&q)==SQLITE_ROW ){
    int mid = db_column_int(&q, 0);
    bag_insert(&seen, mid);
    if( usesFlags & USESFILE_DELETE ){
      db_bind_int(&ins, ":rid", mid);
      uses_file_append_term(&ins, &nIns, mid);
      db_step(&ins);
      db_reset(&ins);
    }
  }
  db_finalize(&q);
  db_prepare(&q, "SELECT cid FROM plink WHERE pid=:rid AND isprim");

  while( (rid = bag_first(&pending))!=0 ){
    bag_remove(&pending, rid);
    db_bind_int(&q, ":rid", rid);
    while( db_step(&q)==SQLITE_ROW ){
      int mid = db_column_int(&q, 0);
      if( bag_find(&seen, mid) ) continue;
      bag_insert(&seen, mid);
      bag_insert(&pending, mid);
      db_bind_int(&ins, ":rid", mid);
      uses_file_append_term(&ins, &nIns, mid);
      db_step(&ins);
      db_reset(&ins);
    }
    db_reset(&q);
  }
  db_finalize(&q);
  db_exec_sql(blob_str(&ins));
  db_finalize(&ins);
  blob_reset(&ins);
  bag_clear(&seen);
  bag_clear(&pending);
}