docs: def posify_mask_indexer(indexer):
      """Convert masked values (-1) in an indexer to nearest unmasked values.
  
      This routine is useful for dask, where it can be much faster to index
      adjacent points than arbitrary points from the end of an array.
  
      Parameters
      ----------
      indexer : ExplicitIndexer
          Input indexer.
  
      Returns
      -------
      ExplicitIndexer
          Same type of input, with all values in ndarray keys equal to -1
          replaced by an adjacent non-masked element.
      """
      key = tuple(_posify_mask_subindexer(k.ravel()).reshape(k.shape)
                  if isinstance(k, np.ndarray) else k
                  for k in indexer.tuple)
      return type(indexer)(key)