private final Object[] help_copy( Object[] helper ) {
    // Read the top-level KVS 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).
    Object[] topkvs = _kvs;
    CHM topchm = chm(topkvs);
    if( topchm._newkvs == null ) return helper; // No copy in-progress
    topchm.help_copy_impl(this,topkvs,false);
    return helper;
  }
Returns the value to which the specified key is mapped, or {@code null}
 if this map contains no mapping for the key.
 <p>More formally, if this map contains a mapping from a key {@code k} to
 a value {@code v} such that {@code key.equals(k)}, then this method
 returns {@code v}; otherwise it returns {@code null}.  (There can be at
 most one such mapping.)
@throws NullPointerException if the specified key is null
jctools-core/src/main/java/org/jctools/maps/NonBlockingIdentityHashMap.java