I notice that the file teletext/saa5050-semigraphic-noncontiguous.yaff has an blank line (line number 55). This causes monobit-convert to abort with an error message:

$ monobit-convert saa5050-semigraphics-noncontiguous.yaff asdf.yaff
ERROR: tuple index out of range

I'm not sure why that happens. YAFF is supposed to be for humans to view and edit, so it'd be good if the format allowed for human imprecision. Perhaps ANTLR (or flex/bison) ought to be used to robustly parse YAFF files?