code: private void store(int bidx, IQueryAtom queryatom) {
          ++sptr;
          stack[sptr].bidx = bidx;
          stack[sptr].iter = null;
          if (queryatom != null)
              stack[sptr].atom = queryatom;
          else
              stack[sptr].atom = null;
      }
docs: Store the specified bond index and mapped query atom (optional)
  on the stack.
  @param bidx bond index
  @param queryatom query atom - can be null