* Indentation-aware parser
Figure out if `bnfc` can support the idea about block-scoping
according to non-decreasing indentation levels.  Examples:

    -- This is a
       comment.

    id
      =  x
      -> x

    id 2

Here the "--" in column 0 starts a comment block which ends the next
time we write something in column <= 0.  Which we do in column 4.
Similarly the binding that starts at row 4 (with the introduction of
an identifier) ends in row 8.