private void help_copy( ) {
    // Read the top-level CHM only once.  We'll try to help this copy along,
    // even if it gets promoted out from under us (i.e., the copy completes
    // and another KVS becomes the top-level copy).
    CHM topchm = _chm;
    if( topchm._newchm == null ) return; // No copy in-progress
    topchm.help_copy_impl(false);
  }
Auto-boxing version of {@link #replace}.
jctools-core/src/main/java/org/jctools/maps/NonBlockingHashMapLong.java