'Detects the block fold level.
The default implementation is based on the block **indentation**.
.. note:: Blocks fold level must be contiguous, there cannot be
a difference greater than 1 between two successive block fold
levels.
:param prev_block: first previous **non-blank** block or None if this
is the first line of the document
:param block: The block to process.
:return: Fold level'

def detect_fold_level(self, prev_block, block):

    raise NotImplementedError
