def METHOD_NAME(istream, byte_offset, block_shape, field_idx):
    """retrieve a specific block (ONE field) from a datfile"""
    # compute byte size of a single field
    field_shape = block_shape[:-1]
    fmt = ALIGN + np.prod(field_shape) * "d"
    byte_size_field = struct.calcsize(fmt)
    istream.seek(byte_offset + byte_size_field * field_idx)
    data = np.fromfile(istream, "=f8", count=np.prod(field_shape))
    data.shape = field_shape[::-1]
    return data.T