docs: @Override
    public Set<TypeK> keySet() {
      return new AbstractSet<TypeK> () {
        @Override public void    clear   (          ) {        NonBlockingHashMap.this.clear   ( ); }
        @Override public int     size    (          ) { return NonBlockingHashMap.this.size    ( ); }
        @Override public boolean contains( Object k ) { return NonBlockingHashMap.this.containsKey(k); }
        @Override public boolean remove  ( Object k ) { return NonBlockingHashMap.this.remove  (k) != null; }
        @Override public Iterator<TypeK> iterator()   { return new SnapshotK(); }
      };
    }