code: private InsertStack getInsertStack()
      {
          Object x = _insertStack.get();
          InsertStack g = null;
          if (x != null)
          {
              g = (InsertStack) x;
              g.reset();
          }
          else
          {
              g = new InsertStack(this);
              x = (Object) g;
              _insertStack.set(x);
          }
          return g;
      }
docs: Find an InsertStack for use by the current thread.
  
  <p>Allocation of an InsertStack is more expensive than serial
  reuse.  This is a very cheap form of pooling done by attaching an
  InsertStack to each thread that calls insert.</p>