File slicing

NB. blocks: given a filename, answers a list of code block start
NB. and stop lines. MUTATIONS: body, lines
NB. pairs = I.>cblk each LF chopstring 1!:1<'smallfile.md'
NB.
blocks =: monad define           NB. 3 : 0
body  =: 1!:1<y                  NB. body is raw file, global scope
lines =: LF chopstring body      NB. lines now scanned, global scope
pairs =. I.>cblk each lines      NB. consecutive start/stops, could be uneven
bump  =. 2|#pairs                NB. bump is 1 for odd pairs allowing us to
(bump+#pairs){.!.(#lines) pairs  NB. fill with the max line ensuring even pairs
)