docs: public static void decrefPlanFragmentById(long fragmentId) {
          // skip dummy/invalid fragment ids
          if (fragmentId <= 0) return;
  
          FragInfo frag = null;
          synchronized (FragInfo.class) {
              frag = m_plansById.get(fragmentId);
              // The assert that used to be here would fail in TestAdHocQueries when it
              // re-initialized the RealVoltDB, clearing the m_plansById before
              // all SQLStmts were finalized. Maybe that's just a "test bug" that would be
              // better fixed with some kind of test-only cleanup hook?
              // OR It's possible that this early return is covering for a minor bug.
              // Maybe SQLStmt.finalize is calling this method when it shouldn't?
              // Maybe that's because the SQLStmt site member should be null in more cases?
              //assert(frag != null);
              if (frag == null) {
                  return;
              }
              if (--frag.refCount == 0) {
                  // The disused fragment belongs in the LRU map at the end -- at the current "ticker".
                  // If its lastUse value is 0 like a new entry's, it is not currently in the map.
                  // Put into the map in its proper position.
                  // If it is already in the LRU map (at a "too early" entry), just set its lastUse value
                  // as a cheap way to notify evictLRUfragment that it is not ready for eviction but
                  // should instead be re-ordered further forward in the map.
                  // This re-ordering only needs to happen when the eviction process considers the entry.
                  // For a popular fragment in a small or stable cache, that may be after MANY
                  // re-uses like this.
                  // This prevents thrashing of the LRU map, repositioning recent entries.
                  boolean notInLRUmap = (frag.lastUse == 0); // check this BEFORE updating lastUse
                  frag.lastUse = ++m_nextFragUse;
                  if (notInLRUmap) {
                      m_plansLRU.put(frag.lastUse, frag);
                  }
              }
          }
      }