static block_header<size_type> *block_header_from_value(T *value)
 {  return block_header_from_value(value, sizeof(T), ::boost::alignment_of<T>::value);  }
 static block_header *block_header_from_value(T *value)
 {  return block_header_from_value(value, sizeof(T), detail::alignment_of<T>::value);  }