code: func (tx *Tx) page(id pgid) *page {
  	// Check the dirty pages first.
  	if tx.pages != nil {
  		if p, ok := tx.pages[id]; ok {
  			return p
  		}
  	}
  
  	// Otherwise return directly from the mmap.
  	return tx.db.page(id)
  }
docs: // page returns a reference to the page with a given id.
  // If page has been written to then a temporary buffered page is returned.